steam pipe failure investigation report

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NASA Langley RNASA Langley RNASA Langley RNASA Langley Reeeesearch Centersearch Centersearch Centersearch Center

Langley, Virginia Langley, Virginia Langley, Virginia Langley, Virginia

Spider topped out against guide Slip misaligned in

Exp. Joint

Anchor Being Pulled From Ceiling

n

Tunnel Floor

No Elbow or Joint

8” Steam Mai

August 14, 2002August 14, 2002August 14, 2002August 14, 2002

By

Kirsner Consulting EngKirsner Consulting EngKirsner Consulting EngKirsner Consulting Engiiiineering, Inc.neering, Inc.neering, Inc.neering, Inc.

Marietta, GeorgiaMarietta, GeorgiaMarietta, GeorgiaMarietta, Georgia

www.kirsner.orgwww.kirsner.orgwww.kirsner.orgwww.kirsner.org

KIRSNER CONSULTING ENGINEERING, INC 8/21/02,5:53 PM Assessment of Problems in Steam Utility Tunnel No 4-COPIED.doc

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Assessment of Problems in Steam Utility Tunnel No. 4 at the NASA LANGLEY RESEARCH CENTER

Kirsner Consulting Engineering was retained through SVERDRUP Technology to provide a third party assess-ment of the chronic problems related to the steam and condensate lines within Utility Tunnel No. 4 at the NASA Langley Research Center (LaRC). This Report identifies the Problems, then lists Findings which illustrate the root causes of the Problems. The Report was authored by Wayne Kirsner, P.E.

An overview of steam system operations re-lating to Tunnel 4, as understood by the au-thor, appears in Appendix A.

THE PROBLEMS, as distilled from reports by NASA LaRC1 and Sverdrup2, interviews with site personnel, and the author’s field inspections are as follows:

! High Pressure Steam (HPS) pipe- an-chors in Tunnel 4 are being torn away from their anchoring points indicating ei-ther severe piping stress or a lack of structural integrity in the concrete or an-chor bolting system.

! Concrete has spalled off the tunnel ceil-ing and walls during shutdowns while the pipe was contracting3. In a 1997 incident, men were operating the valve shown at right (in Tunnel Extension 3 to Recoup) during a partial ceiling collapse.

! Waterhammer in Tunnel 4 is reported to be prevalent during upsets in Recoup operation when Re-coup is supplying steam to the NASA LaRC campus through Tunnel 44. Water hammer may have contributed to the concrete damage shown at right.

! Standby Boilers in Building 1215 have been “knocked off-line” by downward pressure excursions in steam supplied by the Recoup Fa-cility.

1 Status Briefing “LaRC Utility Tunnel No. 4” by Facilities Engineering2 “Task Order 1162 Steam Line In Tunnel #4 Engineering Assessme3 ibid 4 A recent example cited in NASA LaRC “Status Briefing”1 reports that37 to 350 psig caused water hammer at Buildings 1298, 1154, 1208, an

Photo 1. Anchor adjacent Exp. Jt. 27

and Office of Safety and Mission Assurance, April 26, 2002 nt”

on 30 March 2002, an “accelerated pressure recovery” from d 1209.

Photo 2.


FINDINGS AND ANALYSIS:

I. Anchor--Expansion Joint--Guide combinations are misaligned because of improper anchoring design and installation. Background. In general the 8” steam pipe mounted to the ceiling of Tunnel 4 follows the contour of the tunnel ceiling as the tunnel changes slope to climb over and drop under obstacles in its subterranean path. At each direction change, the steam pipe requires an anchor so that pipe expansion and contraction is strictly linear into and out of slip joints. Slight changes in tunnel slope are numerous --the ’64 Structural Drawings show at least 14 changes of slope not counting two changes at each lateral where the tunnel briefly levels out). For the above reason, among others, it’s important that the Structural Drawings be accurate and the Mechanical drawings faithfully reproduce tunnel slope information so anchors are correctly sited. Neither of these conditions is met in Tunnel 4. The initial edition of the Mechanical Drawings (before changes) in-correctly sited a number of anchors demonstrating a lack of understanding by the original design engineer of what anchors are supposed to do.

Finding 1A. The most serious drawing errors were caught (apparently by a NASA reviewer) and corrected by three series of changes labeled on the Drawings as corrections “a”, “b”, and “c” on the “Alternate Add” Drawings (labeled with an “A” as in M-3A). My field verification showed, for the most part, anchors were correctly sited at changes in tunnel direction although there are some slight changes in pipe direction where anchors were not installed.

Although incorrect placement of anchors was caught, close inspection of the anchors and expansion joints reveals that anchors were improperly laid out and constructed.

Finding 1B. At most pipe slope change locations, it is apparent that pipe slope directional change is not taking place at the anchors alone-- as is prerequisite for proper operation of slip-type expansion joints. In-stead the pipe is bending around the anchor (as represented below), then making the rest of its directional change between the expansion joint stuffing box and slip. Primary guides upstream of the misaligned joints are topped-out or bottomed-out confirming misalignment.

I expect misalignment at thjoint are then, in turn, prevpipe anchoring points.

Specific evidence of directmeasuring the difference i


Exp. Joint


Figure 1. T

Tunnel Floor

3

e joints is causing slips to bind in the expansion joints. The bound expansion enting the joints from contracting to relieve thermal expansion stress between

ional change --and hence misalignment-- at the expansion joints was obtained by n exposed length on the top and the bottom of the slips. Where the top and bottom

ypical Change in Direction of 8” HPS Pipe To Follow Tunnel Elevation


Photos 3& 3. Top and Bottom slip Extension at Jt-4.

measurements do not match, the slips must be cocked in the vertical plane. Table 1 lists the expansion joints where we measured cocked slips. Of the 16 joints inspected, virtually every one sited at a change in steam pipe direction was measurably misaligned. (We did not measure misalignments beyond JT-19)

Why Are Joints Misaligned? Finding IC. The source of the misalignment is a failure to provide a mitered pipe section or mi-tered weld at the slight changes in tunnel slope at which the anchors are sited. The origin of the failure begins with the mechanical drawings. A detail excerpted from M-9 of the 1964 engineer-ing drawings is reproduced below. Although the detail is fairly standard in my opinion, it does not specifically show how to deal with anchoring the pipe at a change in tunnel slope.

Exp. Jt. Top Bottom

JT-1 6-15/16 “ 7-0”

JT-2 8-0 “ 8-1/8”

JT-3 3-3/8 “ 3-0”

JT-4 7-1/4 “ 7-3/8”

JT-13 7-3/16 “ 7-1/16”

JT-14 7-0” 7-3/16”

JT-16 6-7/16 “ 6-5/16”

JT-17 7-7/8 “ 7-6/8”

JT-19 5-1/8” 5-1/4”

< Table 1. (There is no Jt.’s 5, 6, 7)

Fig. 2: ’64 Drawing Detail of Typical Anchor,
Scales 2’-10 ¼” 4 Expansion Joint, Guide Installation


5

The detail would have been improved had it shown a directional change in the tunnel ceiling and instructed the Contractor precisely how to deal with it. Below is an example of how the detail might have been im-proved. The detail shows a change in direction for the pipe and adds an extra anchor to deal with it.

The paucity of the detail notwithstanding, the Contractor should havbe joined without an elbow or mitered joint where slip joints are usetraction. Ditto the engineer who inspected and accepted the final w

In order to confirm in at least one case that no miter exists where the steam main makes a directional change, insu-lation was re-moved at the anchor adja-cent JT 19 to reveal the pipe. While the tunnel and 8” steam main do change slope at this point in the Tunnel, the picture at right reveals there is no mitered elbow or weld evident at t

Finding 1D. The Anchor- Joint-Guide Detail on the previous page ment of the primary guide is too close to the expansion joint to effecYarway—the manufacturer of the expansion joints—the primary guipansion joint. ADSCO and ATS —two other prominent manufacturthe primary guide to be 8 pipe diameters, i.e. 64”, from the expansio

Double Anchor station at Changes in Tunnel Slope

Fig 3: Revised Detail

e known that non-colinear pipes cannot d to accept only axial expansion or con-ork.

he anchor.

contains another oversight—the place-tively guide the slip. According to de should be placed 3 feet from the ex-ers of slip-type expansion joints call for n joint slip. The engineering detail re-

Photo 4.


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produced in Figures 2 and 3 calls for the primary guide to be placed directly adjacent the joint. The photo at right of a replaced joint shows the proximity of the guide to the joint. I conjecture that the design Engineer had in mind using the guide as a support. This, however, is not the function of a guide. Ideally the internal fins of a spider guide should not touch the outer guide sleeve unless the pipe tries to squirm out of alignment.

To improve the installation, the primary guides should be removed to their proper position, a support should be placed in the position of the existing guide, and I would add another support under the body of the stuffing box to remove all bending stress from the internals of the joint5.

Have Joints Actually Bound or Seized? There is visible evi-dence of binding at one expansion joint-- JT-19. It’s slip dis-plays obvious dam-age. The photo at right shows it is scored with ½” long scratches around the circumference of the slip adjacent the stuff-ing box. I expect that these scratches resulted from the bound slip being tugged from the stuff-ing box during a cool down, then, remain-ing exposed as the slip hung- up during the subsequent warm-up.

As might be expected the slip is misaligned--extending 5-1/8” on top and 5-1/4” on bottom.

5 I should note there is one joint which, when replaced, was re-engineered and installed correctly—that’s JT 21.

Photo 5.

Photo 6. Scratched Slip


7

Another indicator of whether or not a joint has failed to compress fully is to measure the length of exposed slip. By comparing the exposed slip length with a calculated prediction of how much the joint should have com-pressed, it can be determined whether or not a joint has fully compressed. This calculation requires a knowl-edge of the slip length with which the joints were supplied. While ordering information from NASA LaRC or the manufacturer is lost or unavailable, the following reasoning allows me to “deduce” the probable slip lengths.

1. The “64 Mechanical Set on Drawing M-17 indicates maximum slip compression of 7.2” for any joint. Therefore, we might expect that slips are 8” in length if all joints were ordered with the same slip lengths6.

2. Yarway’s current literature indicates a flanged expansion joint with an 8” slip rated for 300 psi should meas-ure 33-11/16 inch from flange to flange. The same joint with a 12” slip would measure 41-11/16”. I scale the 400 psi rated joint shown on the M-9 Drawing Detail to be 34-1/4 inches—close to the Yarway specification for a joint with an 8” slip.

(Note, the expansion joints that were actually provided by Yarway have welded rather than flanged ends. The welded joints are 2 inches longer than flanged joints).

3. The Structural Drawings include a detail for imbedding support channels in the tunnel ceiling for the an-chors guides and pipe supports. It repeats the dimensions of the Mechanical Detail on M-9. Thus, it’s not likely that the Mechanical Contractor would chose to deviate from the equipment specified on the Mechani-cal Drawings since the Structural Drawings would have had to have been modified also.

4. To partially confirm the above reasoning, Dan McGowen searched for and found a tag on top of JT 2 joint which read, “Yarway 8” travel 8” Fig 6242-S Gun Packed Joint” which indicates that this joint was supplied with an 8” slip.

The Spreadsheet "Expected Slip Exposure" (page 8) calculates the difference between the average length of slip exposed and the length that should be visible if the joint had fully con-tracted. The table assumes all joints were supplied with 8” slips. An excerpt of the spread-sheet’s results is reproduced at left. It shows-- for the joints we measured7-- the number of inches the joint apparently8 failed to compress. This distance, if it is not being taken up in the joint, must be accounted for by either:

1. pushing anchors away from the joint

2. compression of the 8” pipe

3. crushing gaskets

4. pipe squirm (i.e., column bending)

Assuming a compression of 50,000 pounds exerted by anchors—a generous estimate for “primary” anchor resistance-- I calculate pipe compression could only account for no more than .25” per 100 feet of pipe. As to #3 above, there are no gaskets between flanged joints; all joints are welded (contrary to the Engineer’s Detail). And, as for #4, there is no noticeable pipe squirm which is resisted by the proper placement of intermediate pipe guides. Thus…

Finding IE. It would appear that anchors must be slipping or deforming to make up for the failure of the joints to compress. The photo below at right shows a typical example at Jt. 14.

6 Common practice to avoid mix-ups during installation. 7 We tended to measure the worst looking joints up to JT-19. We found no joints labeled 5,6, or 7. 8 This measurement depends on a number of assumptions: 1) 8” slips actually reveal 8 “ in slip length when installed. (2) The con-tracted did not pre-compress the joint at installation to account for the difference in pipe temperature vs. minimum pipe temperature for which the joint is designed to compensate.

Exp. Jt. Diff in JT-1 1.5 JT-2 2.3 JT-3 2.3 JT-4 3.3 JT-8 - JT-9 -

JT-10 - JT-11 -

JT-12 - JT-13 2.7 JT-14 2.5 JT-16 0.8 JT-17 2.8 JT-18 - JT-19 2.4


8

Finding IF. In all cases for which I took pictures, anchors have been deformed to the right (upstream) as shown in the photo above as well as Photo 1. I believe this indicates that the primary guides are hanging-up and contributing to the resistance to movement of the pipe as it attempts to grow. It also may be an indication that roller supports have increased resistance to movement. The reasoning for this finding is as follows--

Theoretically, for a single-sided expansion joint placed near an anchor, the far anchor must resist a greater sum of forces than the close anchor because the extra force needed to overcome friction is all resisted by the far anchor. The sketch below depicts the forces. The left side of the far anchor from the joint, “A1”, must not only resist the horizontal force of anchor A2, but also overcome the resistance of the supports and any hung-up guides if the pipe is to expand to the left. The force on the left side of Anchor 1 “FA1” then will equal:

FA1,left = 2 x FG,I + Σ Fs+ FG,P + FJ + FA2, right cos θ

So clearly, if support and guide resistance to movement is non-negligible, and as long as the angle “θ” is small:

FA1,left > FA2, right Since, the situation above repeats itself up and down the pipe, (i.e., FA1,right, on average equals FA2,right), the force on the left side of the anchors is, in general, greater than that on the right . The more pronounced this asymmetry is, the greater the force being exerted by guides and supports. Photo #9 at right of the guide at JT 2 is typical of the position of the spiders in the primary guides—they are hard up against the top or the bottom of sleeve.

Photo 7. The black arrows show anchor bolts at the anchor just downstream of JT 14 being deformed to the right, i.e. up-stream toward JT 14.

Photo 8.

FA2 FJ FG,I FG,I

Reaction FA1, left FJ FG,I FG,I FG,P

Fig 4: Asymmetric Anchor Forces Support (typ) Guide (typ)

θ

KIRSNER CONSULTING ENGINEERING, INC 8/21/02,5:53 PM

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Expected Slip Exposure Top Mea-sure

Bottom Mea-sure "in"

Average

Degrees

Cocked

Approx. Distance Between Anchors

Expected Contraction from 70oF

Neutral Position @ in/100' =

**

Amount of Slip Expect-ed to Show at 436oF if 70oF slip Differnence

Exp. in in in φφφφ ft 2.97 8.25 inJT-1 6.94 7.00 7.0 0.4 95 2.83 5.4 1.5 JT-2 8 8.13 8.1 0.9 85 2.53 5.7 2.3 JT-3 3.38 3 3.2 (2.7) 248 7.38 0.9 2.3 JT-4 7.25 7.38 7.3 0.9 144 4.28 4.0 3.3 JT-8 171JT-9 0JT-10 203 6.04 2.2 JT-11JT-12JT-13 7.19 7.06 7.1 (0.9) 130 3.87 4.4 2.7 JT-14 7 7.19 7.1 1.3 124 3.69 4.6 2.5 JT-16 6.44 6.31 6.4 (0.9) 90 2.68 5.6 0.8 JT-17 7.88 7.75 7.8 (0.9) 110 3.27 5.0 2.8 JT-18 128 - JT-19 5.13 5.25 5.2 0.9 182 5.41 2.8 2.4

Definition of φ φ φ φ

How Much Might Pipe Compress if anchors seized? E= 3.00E+07 psi/strain fraction

Expansion/100' for steel for 1' contraction in 100' ε= 0.08%7.2"/250' (from Drawings)= 2.88 in/100 ft so σ= 25000 psiExp Coeff @ 450oF = 7E-06 ft elong/oF-ft for 8" sch 40 Ac= 8.40 s. in.

or " elong/100 ft for (436-70)oF therefore F would = 209,910 lbs to compress pipe 1" in 100'

More likely , since anchors are expected to withstand about 30,000 # of forcecontraction would be limited to, for 50,000# of force about .25"/100'. Beyond .25", Iexpect something else to give!

2.97

at corner

looks ok

σ= Ε εσ= Ε εσ= Ε εσ= Ε ε

Cold at 70oF

cold pipe compressed

Hot at 436oF

hot pipe expanded into Gland

Cold Preset

(Slip Length - Cold Preset) - Expansion

(Slip Length - Cold Preset)

Tan φ = Diff/ D

φ

8φ

Should be a cold preset of ~.25" for 70 -40,which would reduce this figure (ATS allows extra 1" standard extra preset, Yarway says 1/4 inch)

Per Yarway Rep John Todd July 2002


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Fig 5: Tunnel Detail From ’64 Me-chanical Set.

17’

.53 c.f

3’

2.92 c.f m=.22 m=.04

Fig 6: Volume to Plug Depression (reversed orientation from Figure 5)

II. Waterhammer . During 4 days of inspecting conditions within Tunnel 4, no water hammer was heard and no definitive evi-dence of prior water hammer events was noted9 (although it’s plausible that the damage to the ceiling shown in Photo 2 could have resulted from repeated water hammer damage). Nevertheless, according to the reports cited in footnotes 1 and 2 in this Report as well as verbal descriptions by Dan McGowen, I understand that water hammer has occurred during the following circumstances:

! During pressure recovery after a downward pressure excursion by the Recoup Plant

! During the act of closing gate valve within Tunnel 4 that isolate Recoup steam from NASA LaRC.

! During Flooding of Pit C in the shallow trench from Recoup.

There are several features of the design and operation of Tunnel 4 which could make it vulnerable to water hammer in the circumstances listed above. Understanding these features is antecedent to instituting operational and maintenance procedures which will avoid water hammer. The features are:

A. The sharp pipe depression in Tunnel Extension #3. This depression is the lowest point in Tunnel 4. Should traps become inoperative or be overcome, excess condensate from over 1300 feet of 8” pipe in Tunnel 4 will flow to this point. Operating 8” isolation valves under steam pressure on either side of this depression is dangerous if condensate has accumulated at this point.

B. Steam Pressure Excursions by the Recoup Plant

C. Potential for high back pressure in the 2’ high pressure condensate return system (HPC) .

D. Small Drip Legs that have low capacity to store excess condensate

E. Potential Flooding of Recoup Shallow Trench Pit “C”

These conditions and their relation to waterhammer in the steam system are discussed below.

A. Pipe Depression in Tunnel Extension #3 & Adjacent Iso-lation Valves

The 8” steam line in the depression can be completely “plugged” by only than 3.5 c.f. of condensate. (See Figure 6.) If condensate collects in this depression more quickly than it can be drained by the trap located at the de-pression, there will likely be waterhammer. I can foresee three scenarios--

i. Should either of the 8” isolation valves on ei-ther side of the depression (Photo 9 shows one, Photo 2 the other) be closed when the depression is filled, or al-most filled, with condensate, and the condensate is sub-cooled, and the system is under steam pressure, there will likely be a rapid condensation event which re-sults in waterhammer at the valve that is being closed. This event could be powerful enough to endanger the operator.

9 During my inspections, the steam supply from Recoup was valved-off so steam was supplied from Building 1215 only.


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ii. If either of the aforementioned isolation valves is closed for an extended period of time, and the depression partially fills with condensate, con-densate will be “induced” to collect up against the face of the closed valve. If the closed valve is reopened under steam pressure, and the pressure differential between the Recoup system and the NASA LaRC system is such that the collected condensate tends to drain through the valve, there will be a severe waterhammer at that valve—once again endangering the operator.

Workers must be cautioned against operating these valves when there is the possibility of accumulated condensate at the depression in Tunnel Extension # 3. The current procedure of using the 8” valve in Pit B of the shallow trench to isolating the Recoup system from NASA LaRC’s is safer, but not full proof either.

iii. If condensate partially fills the depression when steam is flowing rapidly from the NASA LaRC system toward the Recoup Plant, there would be a steam-flow induced slug collision at the 90o elbow down-stream of the depression toward Recoup. This event is discussed further in the next section.

A precursor for each of these scenarios is the collection of condensate at the depression at a rate faster than it can be drained by the single steam trap located at the depression There are several circumstances which could allow this to happen--

1. trap assembly failure at the depression

2. warm-up condensate load at start-up (or during a pressure recovery)

3. excess condensate production beyond the traps’ capacity to drain it due to:

• loss of insulation, or • submerging of a portion of steam line that drains to the trap at the depression.

B. Downward Pressure Excursions Dan McGowen and his operators relate that sudden steam pressure excursions in the neighborhood of 50 to 100 psi downward then back up are not uncommon when the Recoup Facility is handling the entire steam load for the NASA LaRC campus10. A large and rapid steam pressure excursion can temporarily incapacitate boilers11, disable bucket-type steam traps, and conceivably result in waterhammer. This is because when saturated water undergoes a sudden and large pressure drop, it boils throughout its entire mass. The boiling water or condensate leaves the boiler and traps as flash steam. In addition, residual heat in the boiler’s and trap’s metal mass will continue to boil liquid away until the metal is cooled to the saturation temperature of the lower pressure. What’s more, if the pressure drop is large and rapid, vapor bubbles formed in the liquid can multiply the net volume of the mixture so greatly that it bursts out of its container like coke from a warm bottle after it’s been shaken, then opened. 10 Frequency of pressure losses has been “once or twice per week” according to one operator and “ pressure had gone as low as 100 psig”. The report referenced in footnote 1 describes an excursion to 37 psig. 11 The effect of a large water loss in the boiler is to shut the boiler combustion control off on low-water alarm. In the event of a rapid pressure loss from Recoup, this would happen just when the standby boilers in Building 1215 were trying to start in response to the drop in steam pressure from Recoup. For the boilers to recover after being shut off on a safety, feedwater pumps must make up the lost water and the boiler must go through it’s purge/restart cycle.

Photo 9.


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I can imagine the following scenarios resulting from a drop in steam pressure which, upon subsequent pressure recovery, could result in waterhammer:

i. Water overflows the boiler crown valves due to rapid expansion of the water/vapor net volume in the boiler and comes to rest in a low spot in the steam main.

ii. The steam system cools to the saturation temperature of the lowered steam pressure; then upon resto-ration of full pressure, the warm-up condensate load overcomes the capacity of traps to drain it im-mediately.

iii. Steam traps lose their prime allowing steam to blow through into the high pressure condensate return system. This causes the HPC line to overpressurize. Low-load traps are not able to re-prime. The pressurization of the HPC line restricts the capacity of traps that do re-prime and are attempting to remove condensate.

These scenarios and the likelihood that they are occurring are discussed in more detail below. i. Boiler Overflow Boiler overflow due to pressure excursions by the Recoup Plant—Is it occurring and causing waterhammer at Building 1215 ? The following paragraphs examine this possibility and under what conditions it might occur. (I presume in this analysis that pressure loss can only occur when NASA LaRC boilers are in standby mode and thus unavailable to instantly respond to a pressure drop by the Recoup Plant). a. The first necessary condition for overflow is that steam main pressure falls below that in the boiler. In

Building 1215, Boilers in standby mode are kept warm by steam heating coils in their mud drums. I’m told that the heating coils retrofitted into boiler mud drums maintain boilers at a pressure of 250 psig. In standby mode, then, Recoup pressure must fall below 250 psig before any fluid can exit the stop/check valve crowning each boiler.

b. The calculation below computes the sudden pressure drop at which the water in a boiler at 250 psig would form enough vapor to double the liquid-vapor volume.

• At 250 psig, steam volume is roughly 93 times the saturated water volume. • Therefore if 1.1% of the water in the boiler flashed, the net volume of the liquid-vapor mixture

would double. • To flash 1.1% of the saturated water, where hfg= 820 Btu/#, water must give up about 9 Btu/#, i.e.

saturated liquid temperature must drop about 9oF from 406oF to 397oF. • This temperature corresponds to a saturated steam pressure of 224 psig--a 26 psig drop beyond

250psig. Thus, I calculate that an “instantaneous” pressure drop to 224 psig would cause enough vapor to form to double the volume of the water/steam mix in the boiler. This would almost certainly cause the boiler to overflow if it was very rapid.12 But…

c. How Rapid must the Drop in Pressure be? For water to overflow the boiler, the water pressure drop must be rapid enough so that a substantial portion of the bubbles formed in the body of the water do not have time enough to travel to the water/steam surface and exit the mixture before the water-steam mix overflows the boiler crown valve. I’m told that bubbles rise at a rate of roughly from 1/3 to 3 ft/sec13. If the distance the bubbles would have to travel from boiler mud drum to steam drum surface were 20 feet, a substantial portion of the pressure drop necessary to overflow the boiler would have to take place on the order of 7 to 60 seconds.

12 This calculation is meant to be an illustration of the detrimental effect of a pressure excursion as opposed to a recommendation as to how much of a pressure excursion is acceptable 13 Dr. Peter Griffith of MIT


13

Pressure drops due to upset conditions at Recoup, however, may not be this rapid. A chart provided to me by Dan McGowan that recorded the steam pressure drop from 350 to 200 psig (that eventually went to 37 psig) on March 30, 2002 shows the steepest rate of pressure descent occurring from 340 to 255 psi in what looks to me to be slightly less than 2.5 minutes. This equates to a descent rate of 34 psi/ min. The fall from 250 to 200 psig occurred in 5 minutes at a rate of 10 psi/ min.

I don’t believe this rate is, in general, fast enough to make the standby boilers overflow unless, possi-bly, boilers are overfilled, or steam pressure continues to fall rapidly at lowed pressures. (At lower pres-sure, the ratio of vapor volume to liquid volume grows rapidly so that a smaller pressure drop is necessary to double boiler water/vapor volume).

d. Subcooling. Another necessary condition for condensation induced waterhammer is that condensate be subcooled to at least 40oF below the saturated steam temperature. If in Building 1215, Boilers in standby mode are maintained by the heating coils at a pressure of 250 psig, the corresponding saturated water tem-perature is 406oF. When there’s a pressure drop in the steam system, boiler water will boil and thereby rapidly cool to the saturated temperature of the new steam pressure. To drop to a saturated steam tempera-ture of below 396oF (40oF below the saturation temperature of 350 psig steam), pressure would have to drop well below 221 psig. A sudden drop in steam pressure of this magnitude might not only cause enough water to belch out of Boiler crown valves to fill pipe sufficiently to cause waterhammer, but, in addition, if pressure returned to normal before traps drained the condensate, water would be sufficiently subcooled to support a condensa-tion induced water hammer. Thus, a rapid pressure drop to around 200 psig is a necessary condition for waterhammer.

If water should exit the boilers-- e. Boiler water that is expelled into the 8” header from all four boilers in Building 1215 will flow into the

tunnel and downhill to the nearest local low point. The 1975 Survey of Tunnel No. 4 indicates the tunnel floor reaches a local minimum of -5.90’ in elevation at 943 feet from the Plant—just downstream of where the tunnel first turns parallel to Langley Ave. Figure 7 depicts this low spot schematically.

ii. Warm-Up Condensate Load Overcomes Traps after Pressure Recovery During a downward pressure excursion, wetted steam piping will rapidly cool toward the saturation temperature of the reduced steam pressure as it evaporates residual moisture on the pipe walls and at the bottom of the pipe. If the pressure drop lasts for an extended period of time, heat loss at the reduced pressure through insulation will rewet pipe walls, though at a slower rate. When pressure is restored, condensate will re-form in the process of re-warming the pipe to the restored saturated steam temperature. If the walls already hold as much moisture as surface tension permits, the newly formed condensate will roll toward the bottom of the pipe and, thence, flow downhill to the nearest trap. If trap capacity is overcome by the influx of additional condensate, conden-sate will backup and may overflow the drip leg allowing it to either accumulate, or flow downhill to the next drip leg. Aggravating this prospect is the small capacitance of the drip legs in Tunnel 4. Drip legs are 4” nominal diame-ter and about 12” long with the trap take-off midway up the drip leg. Thus they can hold no more than about .08841 c.f., or, 5# of condensate before overflowing.

The condensate removal capacity of the Armstrong 1811 traps used in Tunnel 4 is 530 #/hr at 300 psid14, or 500 #/hr at 250 psid pressure differential across the trap. The latter capacity is more likely toward the end of the tunnel where backpressure is highest in the condensate return system. 500#/hr = 8.33 #/min.

14 psid=psi differential


14

Figure 7. Accumulation of Water/Condensate in Steam Main Near Building 1215

124’ @ 1.03%

208’

124’@ .73%

-1.34'

0 100 200

500 600 700 800 900 1000

-1.9%

20’ from Plant

-1.6% -0.37%

-.21% slope is normal, i.e. ¼ “ per 10’

943’ from Plant

599’ from Plant

338’ @ -0.08%

16’

475’ from Plant

237’ from Plant

-5.90’

5.9 c.f.

.5 c.f.

Accumulation necessary to cover first low point to 25% height of pipe = 6.4 c.f. = 48 gallons = ~380 lbs of water

Corner at 860’

Distance from Building 1215

Steam Branch to Bldg 1268-A

Isolation Valve

Trap Station

-1.9% Slope of Tunnel Floor

-5.90’Elevation of Tunnel Floor per ’75 Survey


15

The example that follows supposes a pressure excursion from 350 psig to 200 psig, and calculates how much condensate might accumulate at the depression in Tunnel Extension 3 upon re-establishment of 350 psig steam pressure. The calculation assumes the pipe has cooled to the temperature of saturated 200 psig steam.

• From before, 1369 feet of 8” pipe pitches toward the depression in Tunnel Extension #3. 1369 feet15 feet of 8” pipe warming from 200 psig/388oF to 350 psig/436oF will produce:

1369’ * 28.55#/ft *.11 Btu/#-oF * (436 –388oF) / 790 Btu/#-stm = 261 # condensate

Almost all of this condensate will form within a minute of steam pressure rise.

• Within this spans of pipe there are 7 traps. In the first minute, they can drain 8.33#/min each or 58# of condensate in the first minute. In the second minute, assume condensate has bypassed two of the far traps-- 5 of them might drain another 41 c.f. In the 3rd minute assume another 8.33 # of condensate is drained. Deducting 35 # that could can be stored in the 7 drip legs, roughly 261# - 58#- 49# - 35# = 119 # would col-lect at the depression in Tunnel Extension 3. This equates to 2.3 c.f. of condensate at 436oF.

Below is what the depression at Tunnel extension # 3 would look like with 2.3 c.f. of condensate accumu-lated at it:

• All but the top 1.4” of the 8” pipe is blocked by condensate. Steam flow from Recoup into the NASA LaRC system will pick up this blockage, plug the pipe, and can carry it up to 33 feet before the slug disintegrates. 20 feet downstream (i.e., in the direction of steam flow) of the depression is a 90o elbow located in Tunnel 4. This elbow will be susceptible to being struck by the slug under the scenario described in this example.

Presumably, similar arguments can be made at other depressions in Tunnel 4, albeit with recovery from larger steam depressions. There are six local low spots in the run of the steam main within Tunnel 4.

iii. Loss of prime. The situation described in the example above could be worse than portrayed if traps are par-tially disabled by the downward pressure excursion because some lose their “prime” during the pressure drop. Traps that have lost their prime could pressurize the condensate return system making it more difficult for working traps to discharge their condensate. Generally traps will “re-prime” once they’re re-exposed to load, but some traps, because they are very lightly loaded, may not re-prime. Note traps T-12, 6, 9, and 11 in the chart below which each drain less than 100 feet of pipe. I calculate that of these traps, the greatest condensate load is 14.4 #/hr. That’s less than 3 % of trap capacity for the Armstrong 1811. Thus these lightly loaded traps (I did not include them all in the sample in the chart) are good candidates for blowing steam in the HPC system upon a loss of pressure.

15 380 feet of 8” pipe drain to this trap. The average length of 15 traps checked was 191feet.

14’

.36 c.f

3’

1.96 c.f m=.22 m=.04

1.4” Clear

6.6”

Steam from Recoup

~.08 c.f.

Ft. of 8" pipe drained by Traps

-100200300400500600

T-1 T-2 T-3 T-4 T-5 T-6 T-7 T-8 T-9 T-10 T-11 T-12 T-13 T-14 T-29

Ft.

Figure 8.


16

C. Potential Flooding of Pit C in Shallow Trench Pit C in the shallow Trench from Recoup is at the lowest elevation of the trench pipe running between the Recoup Plant Tunnel Extension #3. Dan McGowen reports that it has a propensity to flood during vigorous rains. Inspection of the Pit revealed water lines at a depth half way up the diameter of the 8” insulated steam pipe diameter, and some damaged insulation within the trench leaving the Pit. An electric sump pump in the Pit ejects ground water from the Pit, should it exceed the elevation of the Sump in the Pit. Nevertheless, this pump has not always been able to keep up with the ground water ingress.

Submersion of the steam main in ground water will result in condensate ac-cumulating in the steam pipe at Pit C and poten-tially leading to a water-hammer. This is because, as the insulation gets soaked, it not only loses its insulating value, but al-lows ground water to convect around the sub-merged pipe. Heat trans-fer can soar to 1000 times the unwetted value. Should this occur, con-densation production in the pipe will exceed the capacity of the trap to drain it. The condensate will collect at Pit C until it fills enough of the pipe to be blown downstream.

All the potential waterhammer scenarios I discussed with relation to the depression in Tunnel Extension #3 are applicable to the shallow trench steam system if condensate is allowed to congregate at Pit C. Espe-cially dangerous would be the action of operating the 8”isolation valve in Pit B if Pit C’s steam main con-tains subcooled condensate. A safety feature at Pit C which is helpful is the installation of doubling trap-ping at Pit C.

End of Waterhammer Section

Photo 10. Pit C in Shallow Trench from Recoup


III. Overpressurization of Steam System by Recoup.

I’m told that Recoup saturated steam pressure can rise to as high as 400 psig. At 400 psig, steam temperature is 448oF. The following comments are my thoughts as to weather or not this is a problem for the Tunnel 4 Steam System?

A. Per the ’64 Drawings, Sheet M-20, pipe, valves and fittings were specified to be Class 300. Class 300 translates to a pressure rating of 615 psi at a temperature of 450oF. — so, no problem

B. Original Expansion joints are rated for 400 psi per Yarway— so, no problem.

C. A replacement expansion joint manufactured by ADSCO, model # 301-RJ-8-T-F installed as Jt. 21 is rated for 350 psig per paperwork accompanying the joint. This is a non-standard rating point so I’m skeptical of its validity. That said, according to the manufacturer, the serial # of the joint needs to be checked with ADSCO (716-827-5450) to verify its pressure rating —so potential problem.

D. Inverted Bucket 1811 Armstrong steam Traps are rated for a maximum of 400 psig-- so, no problem. E. Primary Anchors loads (at changes in pipe direction) were probably calculated based on a pressure thrust

due to 350 psig steam. An extra 50 psig of pressure would add about 2500 lbs. to primary anchor force at 90o corners and the ends of the line. 2500 lbs of about 30,000 # total represents an 8.3% increase in anchor force—a potential problem considering the present overstressed condition of the anchors.

Maximum expansion Joint travel was presumably based on 436oF max temperature. The saturated va-por temperature of 400 psig steam is 448oF. I calculate that the maximum expansion of any joint is 7.5 inches by design (This is a corrected figure from the ’69 Drawings which lists a max travel for any joint of 7.2”). With at least an extra half inch to work with, the 8” slips should be able to deal with the extra expansion to 448oF—so, no problem.

F. Boiler Safety Relief Valves could be a problem depending on their set point. The set point, by code, can be as high as 3% above the Boiler’s MAWP which I believe is 400 psig, but safety valves are not neces-sarily set or tested for this pressure.—so, a potential problem which could be eliminated by resetting, testing and certifying safety relief valves.

IV. Vulnerability of System to Life Threatening waterhammer accident. Waterhammer in a steam system should not be tolerated. It indicates a design or operational problem which should be documented, analyzed, and remedied. That said, the steam system in Tunnel Four is constructed to withstand all but the most severe waterhammer without fracturing and releasing steam or condensate. This is because:

1. All system components in the steam and HP condensate system are steel. The system does not contain cast iron components or bellow type expansion joints.

2. In general, all joints, except those to drip legs, are welded. (These too were shown on the drawings as welded). Therefore, there are few flanges in which gaskets could blow out should there be an over-pressurization at the joint.

17 Photo 11. Fracture of 18” Sch 40 Pipe carrying 200psig

Steam


18

Notwithstanding the fact that the construction of the tunnel steam system is tough, the NASA LaRC system is not invulnerable to a deadly waterhammer. This is because the system operates at such high pressure—350 psig. A waterhammer event could potentially have enough force to split schedule 40 pipe or blow out gaskets in the few joints that are flanged. The 18” Sch 40 pipe shown above was blown open during a waterhammer event in a system operating at 200 psig.

Ventilation. A feature which enhances Tunnel 4 safety, besides its robust construction, is its forced ventilation system. Steam fitters caught in a steam rupture are generally burned either by direct contact with hot conden-sate spewing from the break, or the filling of a confined space by steam. The ventilation of the tunnel elimi-nates the latter concern and provides workers a means of escape from steam in the advent of an accident—i.e. upwind. The ventilation system is an important safety feature which should be operated and maintained as such.

Finding IV-A. Some areas of Tunnel 4 have little air movement due to the air-intake condition or damper ad-justment. For example, the air intake grate at the Equipment Access #3 is blocked. Because there is an airlock installed just upstream of this point in the tunnel, there is no apparent air movement downstream of the airlock. Some dampers located in laterals to building which admit air to the tunnel are partially closed to prevent con-densate receivers must freezing in the winter by incoming unconditioned air.

Recommend a project be undertaken to upgrade and permanently maintain the tunnel ventilation system to in-clude fixes such as adding heating coils with low temperature controls to outside air intakes, replacement of broken doorway hardware at airlocks, fan and damper maintenance, and test and balance of airflows in the vari-ous tunnel sections.

The End

KIRSNER CONSULTING ENGINEERING, INC 8/21/02,5:53 PM Assessment of

Problems in Steam Utility Tunnel No 4-COPIED.doc

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APPENDIX 1. Brief Description 0f Tunnel 4 and The Recoup Shallow Trench Steam Systems

Tunnel No. 4 extends about 4900 feet from the Building 1215 Steam Plant to Beyond Building 1202. It con-tains one of four steam distribution mains providing 350 psig saturated steam to the NASA Langley Research Center. Added to the existing steam distribution network in 1965, it serves most of the large research building steam loads on campus. In 1980, the steam and condensate lines near the end of Tunnel 4 were interconnected with the waste heat recovery “Recoup” facility operated by the City of Hampton. The Recoup plant generates between 50,000 and 60,000 #/hr of 350 psig+ steam by burning refuse. Recoup steam feeds into the NASA system through an 8” steam main run in a 2200’ long shallow trench which connects to Tunnel No. 4. The steam main in Tunnel 4 thus carries steam in either of two directions depending on whether it is receiving steam flow from the Recoup Plant. Recoup provides all of NASA’s steam needs (backward through Tunnel No.4 to the steam Plant and then out through other mains) whenever it has sufficient capacity to do so, and, large-draw steam ejectors used in research buildings are not operating. When steam ejectors do operate—primarily on weekdays,—the Building 1215 Steam Plant fires-up Boilers to help meet campus load and insure a steady steam pressure is maintained . When steam is not being produced by Building 1215 Boilers, they are kept warm and on standby via steam heating coils installed in their mud drums. Due to a 15 minute purge cy-cle, they require at least 20 minutes to begin producing steam.

Tunnel 4 also contains a 4” low-pressure condensate return (LPC) line and a 2” high-pressure condensate (HPC) return line which was added in a 1968 change order to the original Tunnel 4 work. The HPC line re-ceives condensate from main “drips” (i.e. trap assemblies) from the 350 psig steam mains and returns it to the steam plant. In 1980 when the Recoup trench was built, the HPC return line was extended to the nearest shal-low trench pit (Pit “F”) to receive high pressure condensate from the trap in that Pit. All other high pressure condensate traps from the main in the shallow pit trench (manholes A through E) drain back to the Recoup fa-cility.

The 4” LPC pipe in the tunnel receives pumped condensate from the buildings served by the Tunnel 4 steam main and delivers it to either the condensate receiver in the Building 1215 Plant or to the Recoup Plant. The destination is determined by the operation of a condensate transfer pump added in 1980 to Building 1215. If the transfer pump is OFF, Building 1215 receives all condensate from Tunnel 4 as well as the rest of the cam-pus. This, however, is the exceptional case. The transfer pump is turned on when Recoup is providing steam (which is the normal situation). If ON, condensate joins other campus condensate being transferred through the 4” line in Tunnel 4 to the Recoup Plant.

KIRSNER CONSULTING ENGINEERING, INC 8/21/02,5:53 PM Assessment of Problems

in Steam Utility Tunnel No 4-COPIED.doc

SUMMARY of Assessment of Tunnel 4 at the NASA Langley Research Center

Finding 1A. The most serious errors in the original 1964 design drawings were caught (apparently by a NASA reviewer) and corrected by three series of changes labeled on the Drawings as corrections “a”, “b”, and “c” on the “Alternate Add” Drawings (labeled with an “A” as in M-3A). My field verification showed, for the most part, anchors were correctly sited at changes in tunnel direction although there are some slight changes in pipe direction where anchors were not installed.

Finding 1B. At most pipe slope change locations, it is apparent that pipe slope directional change is not taking place at the anchors alone-- as is prerequisite for proper operation of slip-type expansion joints. In-stead the pipe is bending around the anchor (as represented below), then making the rest of its directional change between the expansion joint stuffing box and slip. Primary guides upstream of the misaligned joints are topped-out or bottomed-out confirming misalignment.

I expect misalignment at thjoint are then, in turn, prevpipe anchoring points.

Finding IC. The source ofthe slight changes in tunnemechanical drawings.

Finding 1D. The Anchor-ment of the primary guide Yarway—the manufacturepansion joint.

Finding 1X. A number of

Finding IE. It would appjoints to compress.

Finding IF. In all cases foshown in the photo above and contributing to the res


Exp. Joint


Figure 1. T

Tunnel Floor

i

e joints is causing slips to bind in the expansion joints. The bound expansion enting the joints from contracting to relieve thermal expansion stress between

the misalignment is a failure to provide a mitered pipe section or mitered weld at l slope at which the anchors are sited. The origin of the failure begins with the

Joint-Guide Detail on the previous page contains another oversight—the place-is too close to the expansion joint to effectively guide the slip. According to r of the expansion joints—the primary guide should be placed 3 feet from the ex-

joints have not compressed fully as demonstrated in Spreadsheet 1 on page 8.

ear that anchors must be slipping or deforming to make up for the failure of the

r which I took pictures, anchors have been deformed to the right (upstream) as as well as Photo 1. I believe this indicates that the primary guides are hanging-up istance to movement of the pipe as it attempts to grow. It also may be an indica-

ypical Change in Direction of 8” HPS Pipe To Follow Tunnel Elevation

KIRSNER CONSULTING ENGINEERING, INC 8/21/02,5:53 PM Assessment of Problems

in Steam Utility Tunnel No 4-COPIED.doc

ii

tion that roller supports have increased resistance to movement.

Finding IV-A. A feature which enhances Tunnel 4 safety, besides its robust construction, is its forced ven-tilation system. Some areas of Tunnel 4 have little air movement due to the air-intake condition or damper adjustment. For example, the air intake grate at the Equipment Access #3 is blocked.

II. Waterhammer There are several features of the design and operation of Tunnel 4 which could make it vulnerable to water hammer in the circumstances listed above. Understanding these features is antecedent to instituting operational and maintenance procedures which will avoid water hammer. The features are:

F. The sharp pipe depression in Tunnel Extension #3. Operating 8” isolation valves under steam pressure on either side of this depression is dangerous if condensate has accumulated at this point.

G. Steam Pressure Excursions by the Recoup Plant

iv. Water overflows the boiler crown v. The steam system cools to the saturation

temperature of the lowered steam pressure; then upon restoration of full pressure, the warm-up condensate load overcomes the capacity of traps to drain it immediately.

vi. Steam traps lose their prime allowing steam to blow through into the high pressure condensate return system. This causes the HPC line to overpressurize

H. Potential for high backpressure in the 2’ high pressure con-densate return system (HPC) .

I. Small Drip Legs that have low capacity to store excess condensate

J. Potential Flooding of Recoup Shallow Trench Pit “C”

III. Overpressurization of Steam System by Recoup.

The system should not be operated above the current design pressure at this time, but could be in the future after expansion joints are operable, anchor loads recalculated, safety valves reset and re-certified.

IV. Vulnerability of System to Life Threatening waterhammer accident Notwithstanding the fact that the construction of the tunnel steam system is tough, the NASA LaRC system is not invulnerable to a deadly waterhammer. This is because the system operates at such high pressure—350 psig.

steam pipe failure investigation report

Documents

field verification

kirsner consulting

air intake

lowered steam

steam system

saturated

steam pressure

nasa larc