comply with asme code during early design stages

H&rr

Comply with ASME

Code During Early

Design Stages

Compliance with

the ASME Code

during the thermal

design phase

assures that the

final sheUranMube

heat-exchanger

design is accurate

and minimizes lost

time and labor

for making

revisions later.

'. -',:K,iGabriel Aiiriqlesr ,

The classical division between

thermal design engineers and

mechanical design engineers is

becoming blurred. Tools are

now available that enable the design en

gineer to perform an integrated ther

mal/mechanical design calculation.

Traditionally, the process engineer

'would design the heat exchanger to

.meet process requirements and then

transfer the basic exchanger geometry

. ; to a mechanical engineer for detailed

,;•. mechanical design. The mechanical en

gineer would apply construction code

: 'rules, such as those established by

;;ASME and, by the Tubular Exchanger

; Manufacturers Association (TEMA),

.: • and check for overstress conditions

,;;)Caused by., thermal and other stresses

(longitudinal, compressive, etc.). In

. many cases, a change was initiated by

the mechanical engineer that would af

fect the thermal design. If the mechani

cal; engineer recognized this, the change

. would be sent back to the process engi-

; neer for review. This "work-in-

prdgress'rb transfer would be repeated

r..^several times, wasting time and money

3H.ca?Twith each transfer.

^kra • With;; recent improvements in com--^puterized design tools, we now have the

ability to ,do a "once-through" thermal

and mechanical design, resulting in sig

nificant cost savings. A project engineer

with the appropriate tools can now over

see the design process from thermal de

sign to mechanical-design without cost-

.... ly.JbackVand-forth. movement of data.

; v^Y-This procedure can-also, be extended to

vendors, thereby saving a significant

amount of time sending, receiving,

checking, and re-sending design data.

Shell-and-tube

equipment design

A successful shell-and-tube heat-ex

changer design includes many aspects:

thermal design, mechanical design, ex

ternal piping loads, seismic loads, wind

loads, support design, cost estimation,

drawing generation, and so on. The

ASME Code (as well as other construc

tion codes, such as AD-Merkblatter

[Germany], Code Francais de Construc

tion des Appareils a Pression (CODAP)

[France], and BS 5500 [U.K.]) plays an

important role in the completion of

many of these analyses.

Many studies have been conducted

on the thermal optimization of heat ex

changers. For instance, Steinmeyer sug

gests that there are over 500 publica

tions on the subject of heat-exchanger

optimization (1). However, true opti

mization not only encompasses the ther

mal design portion, but also mechanical

design. In selecting an optimum heat-

exchanger design for a particular ser

vice, many issues need to be considered,

including the following.

Geometry. The best configuration

for a given process is sometimes diffi

cult to select. For example, a distillation

column reboiler can have either forced

circulation (using a pump) or natural

circulation (where the fluid density is

the driving force), and within these two

categories, the reboiler can be horizon-

CHEMICAL ENGINEERING PROGRESS • JUNE 1998 • 45

ENERGY TRANSFER/CONVERSION

tal or vertical, with or without a re

movable bundle, and so on.

Vessel shapes also contribute sig

nificantly to the cost of the unit. In

general, flat components (e.g., a flat

head) will be several times thicker

than curved ones (e.g., a formed

head). Therefore, unless there are

reasons to specify flat components

(such as for easier access to inter

nals), it is always better to specify

curved ones (2).

Maintenance. It is important to

consider cost reductions throughout

the life of the equipment. For exam

ple, ease of maintenance is very im

portant in high-fouling environ

ments where frequent cleaning is re

quired. Fouling can often be mini

mized by maintaining high fluid ve

locities and avoiding stagnant fluid

regimes. Other considerations in

clude the selection of the most ap

propriate method for cleaning

(chemical or mechanical, off-line or

on-line, etc.) and establishing

whether the tube bundle needs to be

removed to be properly cleaned (3).

Vibration. Vibration or excessive

velocity can occur even in an ex

changer that has been designed for

optimum operation and maintain

ability. In many cases, the type of

baffling or baffle spacing (or both),

tube layout, or tube dimensions may

have to be modified to avoid these

problems. For example, using more

longitudinal flow will decrease fluid

velocities and reduce the probability

of vibration. These changes require

redesigning the unit to establish the

optimum process design that also

avoids these problems.

Erosion-corrosion. Erosion-cor

rosion can decrease the equipment's

operational life. One method to

avoid excessive erosion is to main

tain fluid velocities below an allow

able maximum. Erosion effects are

also important in corrosion chem

istry. Changes in oxygen concentra

tion, as well as the destruction of

protective layers, are other conse

quences of erosion attack (4).

Suitable materials of construction

and protective covers can be select

ed to minimize corrosion attack and

reduce costs. For example, cost sav

ings of over 50% have been realized

using alloy-clad materials rather

than solid alloys (5). In addition,

care should be taken to avoid exces

sive thermal stresses caused by

welding metals with different ther

mal expansion coefficients.

Mechanical design —

the ASME Code

The ASME Code was established

by the American Society of Me

chanical Engineers at the turn of the

century to standardize boiler design

because of frequent boiler explo

sions. The current edition is the

1995 ASME Boiler and Pressure

Vessel Code. The section of interest

to shell-and-tube heat-exchanger de

signers is Section VIII, Rules for

Construction of Pressure Vessels,

Division 1. This is the primary stan

dard used for vessel mechanical de

sign in the United States.

What can go wrong?

Performing mechanical design

independently from thermal design

can result in an exchanger that does

not perform as expected. Two main

types of problems can occur:

• changes initiated by mechani

cal design that require adjustment to

thermal design; and

• mechanical design criteria that

can significantly increase cost.

Changes that

require more changes

Changes made by the mechanical

designer that necessitate thermal de

sign adjustments include:

1. Tubes that will not fit into the

stated diameter. This can occur be

cause (a) a minimum tolerance is re

quired for mechanical construction

(for example, for TEMA T or S rear

head types), (b) a high tubeside de

sign pressure reduces the available

outer tube limit (OTL) calculated

during the thermal design, (c) the

inside diameter was used for ther

mal design but an outside diameter

criterion had to be used for mechan

ical design due to the product form

(e.g., pipe vs. plate), or (d) the out

side diameter was used for thermal

design and the resulting wall thick

ness calculated in the mechanical

design reduces the number of tubes

that will fit in the shell.

2. Additional tube supports added

to stiffen the rear end of bundles

with floating heads or U-tubes (6).

3. Shellside nozzles that are

moved due to construction require

ments, such as mechanical rein

forcement rules, hub flanges clear

ances, etc.

4. Mechanical design changes,

such as a different exchanger type,

different materials, closer baffle

spacing, or different tube dimen

sions, to accommodate the differen

tial expansion of the shell and tubes

in fixed-tubesheet exchangers.

5. Mechanical design changes to

reduce excessive thermal stresses in

one component due to the large tem

perature gradients, for example, the

tubesheet in multipass units.

Tubes will not fit — TEMA T

or S rear head. If the tubeside or

shellside pressure is such that the in

side floating head is relatively thick,

the tube bundle will not fit in the

original outside diameter (Figure 1).

Floating Head

Shell Cover

I Figure I. TEMA T rear head.

46 • JUNE 1998 • CHEMICAL ENGINEERING PROGRESS

Y <•» n 75 W 3

This . occurs because the floating

head design is very sensitive to

pressure, as it is subject to both in

ternal arid external pressure loads.

Tubes will not fit — O.D. vs.

I.D. If the product form is pipe but

the unit was thermally designed to

use plate, the number of tubes may

not fit. The mechanical portion of

the design software will calculate

the required wall thickness. If this

wall thickness is significant and the

design proceeds inward from the

outside diameter, the resulting in

side diameter may not be large

enough to contain the required inter

nals. This problem can also happen

if the thermal design is based on the

vessel outside diameter. The channel

(tube) side may also control the tube

layout, particularly when the pres

sure differential between the tube-

side and the shellside is consider

able. Special tubesheet construction,

such as stub ends for butt welds,

will add additional clearance that

can also reduce the. area available

for tubes.

Additional tube supports. In

typical removable-bundle designs,

the tubes need to be rigid enough to

avoid sagging. One way to stiffen

the bundle is to add support plates

near the rear head of a floating head

or U-tube exchanger (Figure 2). In

some instances, these additional

supports render the tube surface par

tially ineffective in this area. The

Boating Head

Shell

Support Plate

Shell Cover

I Figure 2. TEMA S rear head with support plate.

■ Figure 3. Inlet nozzles must be positioned properly relative to the impingement;<\ :.'. plate and first baffle (a) to avoid fluid bypass and tube erosion (b). '.VI. a

same situation arises in U-tube. con-!

struction •■ when the baffle span is

greater than recommended to avoid

vibration — additional supports are

added at the rear of the U-tube bun

dle to reduce the unsupported tube

span. Care should be taken to make

the tube surface in these areas as ef

fective as possible without unrea

sonable pressure drop penalties.

Incorrect nozzle location. An

other potential problem involves the

position of the shell nozzles relative

to the baffles. The proper nozzle po

sition with respect to the impinge

ment plate and the first baffle is il

lustrated in Figure 3a.

If a reinforcing pad is required

(discussed later), the shell inlet noz

zle may be shifted (to accommodate

the required clearance between the

pad and the tubesheet), as shown in

Figure 3b. With the nozzle in this

position, the inlet flow is split by

the first baffle and much of the flow.

misses the impingement plate, re

sulting in severe fluid bypass and

potential tube erosion. This can be

avoided by moving the impinge

ment plate and first baffle to the

right so they are positioned correct

ly relative to the impingement plate

and baffle. *

Changes in mechanical design.

If design changes are made during

the mechanical design phase, a ther

mal re-evaluation of the exchanger

may be required. For example,- a

common solution to solve excessive

tube compressive stress in fixed-?

tubesheet exchangers is to add more

baffles. If such action- were. taken,

the previous thermal design results

would be incorrect. ■••■•' : :>

Changes to reduce thermally

induced stresses. All stresses in a

vessel must be within the maximum

allowed by code. In a heat ex

changer, since both sides are at dif

ferent temperatures, there will al

ways be thermal stresses. Depend

ing on the temperature gradient,9 ge

ometry, materials: of construction,

and other factors, these stresses can

be significant. Mechanical changes



designed to reduce thermally in

duced stresses can severely impact

thermal performance.

In the simplest case, an expan

sion joint needs to be added to a

fixed-tubesheet exchanger to absorb

the excessive thermal stresses. If the

stresses are still too high after

adding an expansion joint, a change

in geometry is required.

A floating head or U-tube con

struction may be acceptable if a

fixed-tubesheet unit cannot be used.

Exchanger designs that serve dual

purposes can sometimes be used.

For example, if a fixed-tubesheet

kettle is going to be used and the

stress analysis recommends an ex

pansion joint, the kettle itself can

serve as an expansion joint. Clearly,

dual-purpose construction lowers

capital costs.

Excessive thermal stresses can

also occur in multitube exchangers

where the tubesheet is subject to

large temperature gradients from

pass to pass. In these cases, proper

gasket selection is important to

avoid potential leakage due to metal

distortion.

Mechanical criteria

that affect cost

Mechanical design criteria that

can significantly affect the cost of

an exchanger include:

1. Equipment designed under

"lethal service" Code rules (7).

2. Special materials of construc

tion or construction features re

quired to accommodate very low or

very high design temperatures.

3. Supports that need to be re

designed because of excessive shell

or head buckling due to support

loads.

Lethal service. If a vessel's con

tents could kill, it is the user's re

sponsibility to provide a safe design.

The ASME Code provides rules to

make vessels safer under the label

"lethal service."

Although many substances are

harmful if allowed to escape the ves

sel boundaries, the Code defines

lethal substances as poisonous gases

or liquids that are dangerous to life

when inhaled {e.g., hydrocyanic

acid, carbonyl chloride, cyanogen,

xylyl bromide, and others (8)). How

ever, many other processes could

easily qualify as lethal if somehow

the vessel contents were allowed to

escape. The decision then becomes

one of added cost vs. additional safe

ty features. The equipment user has

the responsibility of labeling the

equipment as "lethal service."

Lethal service designs have the

following requirements:

• vessel butt welds must be fully

radiographed;

• body flanges must be of the

hub type;

• carbon steel material must be

post-weld heat-treated; and

• additional rules concerning

tubes and product forms (such as

seamless rather than welded tubes

[the latter require further testing])

must be adhered to.

Full radiography means to x-ray

every butt-weld seam of the vessel to

verify the quality of the welds. Fig

ure 4 illustrates a hub-type flange

with a butt-weld at the end. In con

trast, a simple ring flange is shown in

Figure 5. Note that the simple ring

flange, although more economical,

cannot be used in lethal service be

cause of the lack of a butt weld.

Welds of the type shown in Figure 5

are referred to as fillet welds, and they

cannot be x-rayed in a meaningful way.

Low-temperature service. In

low-temperature service (typically

lower than -20°F), it is important to

make sure that the metals will not

fail due to loss of impact resistance.

Several tests are required to ensure

that unalloyed ferrous materials will

behave adequately at low tempera

tures. At low operating temperatures

(the specific temperature depends

on the particular material), impact

tests may be required on the materi

als and welds. In some cases, differ

ent materials whose properties are

better at low temperatures may have

to be used. The Code requires each

vessel component to be reviewed,

its minimum design metal tempera

ture (MDMT) calculated, and the

controlling MDMT stamped on the

vessel.

High-temperature service. Met

als have different mechanical

strengths that depend on tempera

ture — strength diminishes with in

creasing temperature. The degree of

strength reduction depends on the

metal chemistry and method of fab

rication. High design temperatures

(typically above l,000°F) can exac

erbate other problems, since greater

thickness will be required for higher

design temperatures.

Furthermore, at high tempera

tures, trying to alleviate one problem

can make another problem worse.

For example, if the material is

■ Figure 4. Body hub flange with butt

weld.

■ Figure 5. Body ring flange with fillet

welds.


.'? Vi/.'. "ft f OA3V13

changed from standard-grade stain

less steel to L-grade to avoid sensi

tized welds (welds with chromium-

depleted areas subject to attack by

strong acids), the vessel material

maximum design temperature de

creases from l,500°F (815°C) to

800°F (427°C) (9), which could

make the material unsuitable for the

original service.

Shell or head buckling due to

support loads. Horizontal or verti

cal supports impose loads on the

walls to which they are attached. Ad

ditional metal, in the form of rein

forcing pads, is sometimes required

to reduce excessive support loads.

Support location will also impact ad

jacent components, such as formed

heads. It may be desirable to move

the supports to a better location. In

vertical vessels, if an expansion joint

is present, the support location is im

portant in order to avoid overstress-

ing the expansion joint/shell/tubes

assembly.

Reducing capital costs

Three examples illustrate steps

that can be taken to minimize capi

tal costs.

MCH/toluene separation using

phenol. Consider a process for the

separation of methylcyclohexane

(MCH) from toluene using phenol,

which consists of a column, reboil-

er, condenser, preheater, and auxil

iary equipment. Total equipment

costs were estimated to be

$857,400, 57% of which is for the

column. Clearly, this is an area

where cost reductions would have a

significant impact. The next largest

cost item is the reboiler, estimated at

$186,400 (22% of the total), followed

by the condenser at $81^700 (10%).

Using integrated thermal/mechani

cal optimization, the cost of the re-

. boiler was reduced by 40%, from

$186,400 to $109,500, and the con

denser cost was reduced by 65%,

from $81,700 to $29,000 (Table 1).

Notice the last reboiler alternative,

which is a vertical reboiler with the

vaporization taking place on the tube-

side — the horizontal shellside-vapor-

ization kettle with finned tubes is

roughly equivalent in price to the ver

tical tubeside reboiler with plain

tubes.

Glycerin/water evaporator.

Differences in material of construc

tion costs can be significant. Con

sider a glycerin/water evaporator

operating under vacuum. If corro

sion were not a problem and carbon

Table 1. Design alternatives for Example 1, MCH/toluene separation using phenol.

Table 2. Effect of material selection on cost in Example 2, glycerin/water evaporator.

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Conclusion

With the advancement of inte

grated tools, the project engineer

can now design shell-and-tube heat

exchangers from beginning to end,

including fabrication drawings. The

advantages of this once-through ap

proach are significant:

• reduced costs because of an

optimized thermal and mechanical

design;

• reduced costs as a result of ap

plying ASME rules early in the pro

ject and minimizing rework later;

• reduced costs associated with

shorter project completion times;

and

• improved cost projections re

sulting from the simulation of

equipment fabrication.

steel could be used on the process

side, the equipment cost would be

60% less than if the unit were built

of the recommended Type 316L

stainless steel. If the process side of

the exchanger were classified as

lethal service, the increase in cost to

meet the more-stringent construc

tion criteria would be 15%; addi

tional costs would be incurred if the

whole unit were to be classified as

lethal service (Table 2).

Platform feed vaporizer/reactor

effluent condenser. Although there

are obvious cases that dictate

whether a fluid should be placed on

the shellside or tubeside to maxi

mize thermal performance, even in

cases that look equivalent it is im

portant to at least consider the possi

bility of switching sides. In this ex

ample, platform feed is vaporized

from a vapor/liquid mixture at about

165 psia and reactor effluent is par

tially condensed at about 433 psia.

Everything else being equal (such as

maintenance requirements, etc.),

switching fluid sides reduces the

cost by 22% (Table 3).

Project completion time

Thermal and mechanical design

integration also reduces the overall

costs of a project by reducing the

project completion time (10). Addi

tional steps, such as drawing gener

ation and vendor interfacing, could

be taken in parallel, thereby further

reducing the project completion

time. This idea of achieving faster

project execution has also been re

ferred to as "fast tracking" (II).

Table 3. Effect of fluid allocation on cost in Example 3,

platform feed vaporizer/reactor effluent condenser.

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comply with asme code during early design stages

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