flow measurement - control globalflow measurement. fall, 2019. ehandbook. table of contents. how to...

FlowMeasurementFall, 2019

eHANDBOOK

TABLE OF CONTENTSHow to specify a control valve 4

Understand valve anatomy, terminology, data sheets and what could go wrong.

How to optimize pumping costs 10

Use a variable-speed pump and minimize valve pressure drops.

Remote control of fractionation 15

Can smart differential pressure (DP) cells be used for custody transfer?

Compressor surge control 18

Deeper understanding and simulation can virtually eliminate instabilities.

AD INDEXAcromag • http://www.Acromag.com 3

Kobold Instruments • https://koboldusa.com 9

Krohne • http://us.krohne.com 14

Endress+Hauser • http://www.us.endress.com 17

eHANDBOOK: Flow Measurement, Fall, 2019 2

www.ControlGlobal.com

Visit www.Acromag.com/SCTO LEARN MORE

You can depend on Acromag for all

your monitoring and control solutions.

New Dual-Channel ProgrammableTransmitters & Signal Splitters

■ Dual channel saves you money

■ Front facing terminal blocks saves you time

■ High performance increases accuracy

Acromag.com

877-295-7066

Linkedin.com/company/acromag

MADE IN USA

ISO9001AS9100

IECEx

http://www.acromag.com

Valve specification begins with the control

loop, which includes the transmitter that

forwards signals to the computer, positioner

and actuator. The control valve assembly

consists of the actuator that opens and

closes the valve, positioner that converts

electric signals to pneumatic signals, and

valve that partially obstructs flow to control

a process parameter.

VALVE ANATOMIESUp close, the positioner has a lot of moving

parts, but it’s typically a box mounted directly

to the valve; it hooks up to the instrument

air supply; supplies the actuator with air to

control valve position; and often provides

diagnostic information for the valve.

Meanwhile, the actuator receives a pneu-

matic signal from the positioner; amplifies

it to overcome process pressure; and pro-

vides a safe failure direction in the event of

signal loss, such as “open, closed, last.” The

two main types of actuators are:

• Diaphragm actuators, in which air pressure

acts on a diaphragm with high surface

area, and it’s opposed by a Fall that dic-

tates fail direction. They’re low-friction;

provide a fast response to small changes

(which account for most changes in a pro-

cess); and have a longer response time for

larger changes. They’re the process indus-

try standard.

• Piston actuators are selected when more

thrust or greater stroke length is needed.

They’re single-acting or double-acting.

While double-acting provides better thrust

and is more precise, it also requires a

How to specify a control valveUnderstand valve anatomy, terminology, data sheets and what could go wrong.

by Eric Lofland



volume tank that needs maintenance, as

well as a locking mechanism to fail in any

position other than last.

The valve body’s primary features include

the process connections, such as flange, NPT

or wafer, which join it to the pipe; stem that

moves the final element to partially obstruct

process flow; and vena contracta, which is the

point where the stream has the lowest flow

area. There are two main design families:

• Rotary valves are modulated by a rotating

stem, and are generally classified by the

design of the closing member. They’re

full-port and free draining, have high

capacity for a given line size, and have a

narrower optimal control range. The main

types include butterfly, v-notch ball and

eccentric plug valves.

• Sliding stem valves are modulated by

moving the stem in a linear fashion. They

have lower capacity for a given line size,

and a wide control range. Their two

varieties are balanced and unbalanced.

Unbalanced moves against process pres-

sure, and is easier to seal. Balanced uses

holes to transfer pressure, and doesn’t

require as much force from the actuator,

but requires a seal between the plug and

the cage.

THE DATA SHEETSo how do users describe what valve

technology they need? They use the data

sheet. Examples here use the ISA’s (www.

isa.org) format, but vendors also provide

data sheets. And, though most have a

similar structure, many client sites have

a preferred format that supersedes the

ISA’s requirements.

We recommend filling out a data sheet by

answering the well-known riddle, “How do

you eat an elephant?” One bite at a time.

This means separating the data sheet by

sections, and tackling each section individu-

ally. The typical sections are:

• Revision block lists people or groups with

information, instrument history, project

history and changes over time. It’s the

best place to start because it’s informa-

tion you already have.

• Process data is a thumbprint of the

application, and should be filled out by a

process engineer or someone else who

knows the process. It helps determine

chemical compatibility, physical design

specifications and sizing. More informa-

tion is better! For example, if you’re sizing

for three flowing conditions, each needs

a unique flow, inlet and outlet pressure.

Ask questions that might not be apparent

about the section, such as, is the service

dirty/corrosive, erosive, toxic or flamma-

ble? You can always remove unnecessary

information when the data sheet is issued.

• Manufacturers and valve type includes

picking a manufacturer from the site’s

approved manufacturer list; consider

the body style (rotary or sliding stem)

that suits the application best; and

find a model type that suits the design



temperature, shutoff pressure, and line

size of your application.

• Actuator is generally selected based on

valve type, and sized by the vendor. A

hydraulic calculation is used based on

valve requirements and minimum avail-

able air pressure. A safety factor of 10%,

25% or 50% is usually included based on

how likely the service is to clog the valve

or inhibit valve movement.

• Positioner is chosen by input type and

desired diagnostics. High performance

is often available for applications where

response time is critical.

• Materials include two types: external and

trim. Requirements for trim are more

stringent as small changes to these

surfaces can drastically affect valve

performance.

• Notes is a great place for information that

doesn’t fit in the body of the data sheet.

This can include testing and certifica-

tion requirements or references to other

relevant documents. However, avoid

installation notes here, or specifications

that won’t be provided to the vendor.

It’s also important for all valve specification

team members to communicate needs and

expectations clearly, as real specifications

generally require compromises in certain

areas to optimize performance.

SIZING THEORYFlow isn’t all that defines flow capacity

(Cv) or contributes to selecting the right

valve. It’s only part of the overall puzzle. Cv

defines flow capacity required by process

conditions (Figure 2).

Most manufacturers supply sizing software.

Users enter process variables, and the

software outputs the Cv for each flow con-

dition. Users look at the flow Cv curve to

pick an appropriately sized valve. The curve

plots Cv travel % for a value that’s differ-

ent for each valve, and supplied by most

vendor software.

Placing the operating points at favorable

control points on the Cv curve is important.

Plus, characteristics vary by valve type.

WHAT COULD GO WRONG? PLENTYThe three primary issues that can adversely

impact valves and specifying them are

material such as corrosion and erosion,

FLOW CAPACITY EQUATIONSFlow is not all that defines flow capacity (Cv) required by process conditions, but these equa-tions do. Most manufacturers supply sizing soft-ware, users enter their process variables, and the software outputs the Cv for each flow condition. Users then look at the flow Cv curve to pick an appropriately sized valve. Source: Ambitech

Q = flowFP = piping geometryΔP = valve pressure dropGL = liquid specific gravityZ = compressibilty factor of gas

N = empirical constant (manufacturer dependent)

P1 = valve inlet pressureT1 = valve inlet temperature Gg = gas specific gravity

∙Cv (liquid) = Q

NFP ΔPGL

Q

NFP P1 ∙ΔPP1

GgT1 Z

Cv (gas) =



capacity such as high and low Cv require-

ments, and phase change such as flashing,

cavitation and two-phase flow.

Corrosion solutions can be found by: con-

sulting a material compatibility table;

consulting a materials expert in your orga-

nization; investigating other applications for

a similar service; and considering a corro-

sion-resistant alloy such as Hastelloy, Monel

or titanium-based alloys.

Erosion solutions include: hardening wetted

parts with martensitic alloys or stellite

facing, installing a filter upstream; avoid-

ing restricted trims; using a rotary design

to maximize surface area and minimizing

path changes.

High Cv applications have high through-

put with low pressure drop, and are often

found in pump suction or as an add-on to a

system already hydraulically stretched. The

solution for high Cv is to have the largest

vena contracta possible, and use an open

port, which usually means rotary valves.

Low Cv applications have low throughput

and high pressure drop, are often injection

or purge applications, or in level settings

on highly pressurized vessels. The solution

for low Cv is to use the smallest vena con-

tracta, often to employ restricted trim, and

to recognize that these applications are

sensitive to plugging and that there are lim-

ited anti-cavitation options.

Turndown = Cv max / Cv min are addressed

by solutions available for high or low Cv

applications, though it’s difficult to do both.

Talk to the expert users of the process, and

find out what’s most imporant to them if

you find no valve exists that can control at

both the minimum and maximum points.

Phase changes include flashing, cavitation

and two-phase inlet flow. These involve big

physical changes that can cause a lot of

stress. Phase change risk factors include:

when the initial phase is liquid, vapor pres-

sure that’s close to inlet or outlet pressure,

or a pressure drop that’s a large percent-

age of inlet pressure. When fluid moves

through a valve, flow remains constant

through the system.

Flash occurs when flow goes in as a liquid

and out as a vapor, which can cause a lot of

damage due to hydrodynamic stress. While

there’s no way to reliably predict the extent

or rate of damage, reducing the velocity

of the stream can help, as can redirect-

ing the impact to non-valve surfaces, and

hardening all control surfaces. Frequently,

this means angle valves or rotary valves in

reverse flow.

Cavitation is a liquid-vapor-liquid phase

change that can be very violent. Solutions

include beefing up with restricted or hard-

ened trims like stellite, which can mitigate

damage, but won’t prevent it. Users can

also deploy special anti-cavitation trims



to prevent cavitation, such as: cage type

with small holes, though they can plug and

may need service, or notch type that uses a

series of notches to redirect flow. The idea

is to stage the pressure drop, so cavitation

doesn’t occur, but this also means restricted

flow profiles and minimum flows.

Two-phase flows come in two types:

gas-liquid that contains two different com-

ponents in two phases such as air and

water, and vapor-liquid that contains one

components in two phases such as steam

and water.

No method exists to analytically calculate

Cv in a two-phase system. Consequently,

the first rule for sizing a control valve for

two-phase flow is—don’t! However, if you

must size for two phases, then think happy

thoughts, ensure you have flow data on

both phases, approximate the Cv by adding

the Cv of each phase, and in vapor-liquid

phases, include overcapacity to account for

possible process upsets.

Eric Lofland is senior engineer in the Instrumentation

and Controls Dept. at Ambitech Engineering Corp.,

which is a Zachary Group company. He can be reached

at [email protected].



www.koboldusa.com 1801 Parkway View Drive, Pittsburgh, PA, 15205 1-800-998-1020

• Durable Metal Housing • 2 Configurable Outputs• Superior Measuring Accuracy• Bi-directional Flow Measurement• Extremely High Turndown Ratio

MISMIM+ Revolutionary Economical Magnetic Inductive Flowmeters

HPC A Worldwide First in Coriolis Technology Architecture

The MIM magmeter delivers a revolutionary design for measuring and monitoring the flow and temperature of

conductive liquids in pipes. The compact design offers exceptional features and functions, at an economical price.

Engineered to exceed the competition, the MIM triumphs with: a superior measuring accuracy, four times the turndown ratio, easy onsite programming, batching functionality, and bi-directional flow measurement.

The MIM is built to last, with a rugged stainless steel body. The display is both configurable and rotatable. The MIM is an ideal solution for a variety of applications; with ranges from 0.48...48 GPH to 0.4...90 GPM, temperatures to 284 °F, and pressures up to 230 PSI.

The MIS is a full bore electromagnetic flowmeter featuring the same unique and versatile electronics module and is suitable for a wide range of applications.

New Products from KOBOLD Instruments: Setting the Standard for Cutting Edge Technology in the Field of Instrumentation

The HPC breaks the barriers of low-flow measurement for Coriolis flow meters. Most low-flow options employ a single tube design where external interference increases dramatically, requiring costly decoupling. Another challenge most low-flow options face is that the weight influence of the sensor coils compared to the pipe diameter limits the potential design size. The

revolutionary HPC employs lightweight magnets that are mounted onto the pipes themselves. This provides

the sensor with significantly noise-reduced and predictable dynamic behavior, capable of functioning at higher frequencies, further decoupling the sensor‘s measurement from any external vibrations.

• Batching Function is Standard• Grand and Resettable Totalizer• Color, Multi-parameter Touchscreen• Display Rotates in 90° Increments• Intuitive Set-up via Touch Keys

• Unique Coriolis Technology • For Very Low Flow Rates• Rugged Flow Body

• Exceptional Accuracy• Vibration Resistant• Modular Mounting Concept

Like the MIM, the MIS can accomodate all flow directions due to the rotating digital TFT display screen. The rugged flow bodies are made of cast steel. The MIM and MIS feature a convienent IO-Link, especially useful for Industry 4.0 compliance. Analog, frequency, and pulse outputs are standard along with alarm, batching, and totalizing features.

The HPC integrates up to 4 sensor coils which increases the resolution accordingly. HPC sensor coils are mounted between the pipes, not on them. This new concept delivers an extremely small meter with exceptional accuracy and resistance to external interference.

https://koboldusa.com/products/flow/flow-measurement-without-moving-parts/

Q: Delivery of transmix fuel to two sepa-

ration towers is being upgraded. Coriolis

meters were installed and are currently

functioning. Two PID flow control loops

currently maintain flow. The transmix

liquid passes through a series of heat

exchangers on its way to the towers.

It’s estimated that between the friction

losses in the heat exchangers and the

elevation change, the head loss is on the

order of 30 psi.

The Coriolis meters require minimum pres-

sure to function. Now, the pump can’t meet

the minimum pressure for both towers in

service, so only one tower is in service.

A new pump and VFD will be installed.

Two pressure transmitters will also be

installed—one right after the pump, the

other up by the towers just before the

split to the two flowmeters. There’s an

estimated 30- to 45-second lag between

a change in pump speed and an observed

change in pressure at the flowmeters.

The plan is to use a new PID pressure loop

to maintain the pressure just downstream

of the new pump by adjusting the VFD.

The second transmitter up by the flowme-

ters will be used with logic that helps avoid

low pressure at the flowmeters. If pressure

up there falls below a threshold, logic will

put the flow loops in manual and start clos-

ing the valves to build pressure back up.

The question is whether this design concept

is sound and valid, or will there surely be

interaction? Or maybe it will work, but we

have to try it and fine tune it?

William Love. [email protected]

How to optimize pumping costsUse a variable-speed pump and minimize valve pressure drops.

by Béla Lipták



A: This is a valuable question because it

applies to all pumping system applications.

The important point to remember is that

one can’t independently control both the

pressure and the flow of liquids flowing in

a pipeline because pressure is dependent

on flow. Therefore, the proposed control

system is unworkable (Figure 1).

The relationship between flow and pressure

is determined by the pump and system

curves, and the operating pressure (sum of

static head and friction loss) is at the point

where the two curves cross (Figure 2).

Liquids are incompressible, so there is no

dead time between flow and pressure.

Therefore, the cited dead time of 30-45

seconds is in error. Cascade control can’t

be used because the flow and pressure time

constants are nearly identical, while for

good cascade control, the slave must be an

order of magnitude faster than the master.

If you want to save pumping energy, you

can control the speed of the variable-speed

pump using a valve position controller

(VPC). The VPC minimizes the valve pres-

sure drops by opening the valve that is

most open to 90%, all the time. As shown

in Figure 3, if the system was designed, so

that normal operation would require an

average speed of about 50%, you’ll require

only 13% of the horsepower of using a con-

stant speed pump (100% speed in Figure 3).

For a variety of other pump optimization

schemes, read the pump chapter in the 4th

edition of Volume 2 of my handbook.

My recommendation for your system is

shown in Figure 4. Note that I also added

PC

FC

PT

FC

Lowpressure

logic

Transmixfuel Delete

Head

Capacity

Pump head—capacity curve

System head—capacity curveFriction andminor losses

Total statichead

OPERATING POINTFigure 2: The system curve determines the rela-tionship between flow and pressure. The operat-ing pressure will be at the point where the pump (dotted) and the system (solid) curves cross.

AS-PLANNED SYSTEMFigure 1: Delete pressure controls because pres-sure and flow can’t be independently controlled. Once flow is controlled, the pressure is deter-mined by the system.



a pressure safety valve (PSV) to protect your system

against overpressure.

Millions of dollars could be saved in various industrial

applications if all pumping energy costs were minimized

the same way as shown in Figure 4.

Béla Lipták, [email protected]

FIGURE HEADFigure 3: If a valve position controller (VPC) is added, this inde-pendent control loop can keep the most open valve always 90% open by modulating the pump speed. If the pump is oversized, the operating cost savings can be substantial.

F2 F1

Head or pressure

Flow

100% speed

67% speed

P1

P2

Thro

ttled

Unthrottled

RECOMMENDED CONFIGURATIONFigure 4: When optimized, the the valve position controller (VPC) always minimizes the system pressure drop by keeping the most open valve at 90% open.

Transmix fuel

FC FCVPTVPT

VPC

PSV

>

SP: 90%

A: I suggest that you review

the design of the centrifugal

pump. With a given impel-

ler, the pump will pump a

volume of liquid at a given

head at a given speed. If

the pump can’t provide the

required head at a given

speed, then increasing the

rotational speed at the VFD

will increase the head. The

governing law of physics is

the Pump Affinity Law:

1. The flow (GPM) varies pro-

portionally with the change

in speed. This means that

twice the speed is twice the

flow. One-third speed is one-

third the flow.

2. The pump head (pressure)

varies with the square of the

change in the speed. Dou-

bling the speed generates

four times the head. At 80%

speed, the head generated

is 64%.

3. The power requirement

(horsepower or kilowatts)

varies by the cube of the

change in speed. Twice the

speed would consume eight

times the power; half the

speed would require one-

eighth the power to drive

the pump.



A speed change at the pump must imme-

diately appear as a pressure increase at the

flowmeter in this liquid-filled system. The

30- to 45-second delay is just not physically

possible; the pumped liquid has to go some-

where. If the VFD responds to the speed

change properly, the pressure increase

should be instantaneous. Many VFDs have

filters installed on speed change to prevent

too rapid a speed change that might result

in pipe hammer. However, 30-45 seconds

seems out of reason.

In a filled piping system, VFD speed change

will result in an immediate change in flow

rate and pressure head unless a ramping

function is configured or the pipe isn’t filled

with liquid. What you describe isn’t possible

with a filled pipe.

Dick Caro, [email protected]



Accurate, reliable and durable flow measurement in chemical processesOPTIFLUX 5000 series – technology driven by KROHNE• ½ to 12“ Electromagnetic flowmeters with ceramic measuring tube for corrosive

or abrasive fluids

• Vacuum and over pressure resistant, leak proof with fused-in Cermet electrodes

• –40 to +180 °C (–40 to +356 °F) , thermal shock resistant materials

• Hazardous area approved

fact

More facts about the OPTIFLUX 5000: us.krohne.com/optiflux5000

products solutions services

gal/h

ft/sft3/h

°F

http://us.krohne.com/optiflux5000

Q: I work as a product manager for Emer-

son. In the column on custody transfer

(July ’18, p. 43, www.controlglobal.com/

articles/2018/why-can-a-dp-flowmeter-

be-used-for-gas-but-not-liquid), you

stated that the DP flow turndown on

liquid service is 3:1 to 4:1, and does not

have the ability to compensate for dis-

charge coefficient. Rosemount’s 3051SMV

with Ultra for Flow dynamically compen-

sates for changes in discharge coefficient

22 times per second, and is capable of

±1% of mass flow measurement over a 14:1

turndown on flow.

Ben Goulet, [email protected]

A: My column discussed standard DP cells—

you are right that smart ones provide 200:1

P or 14:1 flow rangeability.

Concerning discharge coefficient compen-

sation, which is provided to correct for gas

expansion, and concerning thermal expan-

sion factors in the DP mass flow equation, it

should be emphasized that the value of nat-

ural gas is a function of its composition and

heating value, and pressure and/or tempera-

ture don’t detect either of them, no matter

how often the calculation is performed.

In addition, the DP cell doesn’t measure

mass flow (nor volumetric flow); it measures

the square root of the pressure differential

across a flow element. Also, the DP cell is

only one component in the flow detection

loop, and therefore it’s misleading to imply

that the DP accuracy and the flow measure-

ment accuracy are the same. They are not.

The flow measurement error is the sum of

Remote control of fractionationCan smart differential pressure (DP) cells be used for custody transfer?

by Béla Lipták



all other loop component errors, including

installation ones.

You say your flow measurement error is

±1% without stating ±1% of what. If you are

claiming a ±1% AR (actual reading) accuracy

at minimum flow (full flow divided by 14),

then you’re claiming that your detector’s

full scale flow accuracy is 1/14 = ±0.071%

FS. If that’s what you claim, that means that

in terms of P, you’re claiming an accuracy

of 1/200 = ±0.005% FS, which is obvi-

ously unrealistic.

On the other hand, if your ±1% flow accu-

racy claim refers to ±1% FS, that error at

minimum flow corresponds to an error

of ±14% AR, which makes the measure-

ment useless.

Béla Lipták, [email protected]

A: Regarding the use of DP flowmeters for

custody transfer, my advice has always

been, don’t. Aside from the fact that

dP flowmeters were never intended for

measuring mass flow, there’s the frequent

error of installation and wear of the orifice,

which doesn’t maintain a sharp edge.

For the ISA CAP course, I teach that an

orifice/DP flowmeter is great for control

purposes, since even when incorrect, it is

consistently incorrect and highly useful for

flow control. I’ve even used orifice/DP for

measuring steam flow in an energy/mass

balance situation on a paper machine, but

that’s far from custody transfer.

However, even with pressure and tempera-

ture compensation, it just isn’t accurate

enough for custody transfer. I recommend

a Coriolis flowmeter, or for some liquids in a

low-flow situation, a positive displacement

pump. In class, I use an example of meter

accuracy vs. tank level measurement for

custody transfer. Only a Coriolis flowmeter

can rival the accuracy of custody transfer

through tank level measurement.

Dick Caro , ISA Life Fellow, [email protected]



Proline 300/500 - Flow measuring technology for the future

• Added value throughout the entire life cycle of your plant, based on decades of experience in safety-related application

• Entirely developed according to SIL (IEC 61508). Maximized plant safety and availability due to unique features – such as webserver, WLAN, WirelessHART, Industrial Ethernet, or Heartbeat Technology with comprehensive diagnostic and verifi cation functions

• Multifunctional transmitters – combinable with all tried-and-tested Promass and Promag sensors

• Seamless system integration via HART, PROFIBUS PA/DP, FOUNDATION Fieldbus, Modbus RS485, EtherNet/IP and PROFINET

We understand you need insightful process information to help you run your plant e� ciently.

You make confi dent decisions backed by process data and a complete portfolio of services and solutions to support you.

Do you want to learn more?www.us.endress.com/proline-simply-clever

MEASURED VALUE+ ADDED VALUE

http://www.us.endress.com/proline-simply-clever

Axial and centrifugal compressor control

is exceptionally challenging due to the

extraordinary speed and severity of prob-

lems, and the extreme consequences in

terms of plant safety and performance. The

fastest and most dangerous phenomenon

is compressor surge. An axial or centrifugal

compressor can reverse flow in 0.03 sec-

onds, going from a large positive flow to

a large negative flow. Often, the negative

flow is not measurable by flowmeters, leav-

ing the actual situation to your imagination.

If you knew exactly what was happening,

it would be even scarier, motivating you to

seek greater understanding and prevention

of the problems. Presently, we tend to rely

on companies specializing in surge control

to protect you and your plant, but what

happens if something is not quite right in

the middle of the night, causing a surging

fright and piping to take flight?

UNDERSTANDING COMPRESSOR SURGEThe surge point on the compressor map

(typically a plot of compressor pressure rise

versus suction flow) is the point where the

slope of the characteristic curve becomes

zero. Each characteristic curve corresponds

to a particular speed or inlet guide vane

position. The blue plot in Figure 1 shows the

compressor characteristic curve seen and

unseen. Compressor manufacturers often

don’t show the compressor characteristic

curve to the left of the surge point, creating

mystery and vulnerability.

A first-principle model has provided the

knowledge of how negative and positive

Compressor surge controlDeeper understanding and simulation can virtually eliminate instabilities.

by Greg McMillan, Chris Stuart and Thomas Hildebrand



feedback occurs from the

sign and magnitude of

slopes seen and unseen in

the compressor character-

istic curve. The negative

slope of the curve to the

right of the surge point

provides some nega-

tive feedback to help

with stability. As the flow

decreases, the pressure

rise increases, creating a

greater downstream valve

pressure drop and possibly

flow. If the downstream

valve position continues

to decrease, the operating

point proceeds to walk to

the left from point A along

the characteristic curve.

When the operating point

reaches the zero-slope

point, it jumps in about 0.03

seconds to a negative flow,

signifying the beginning of

the surge cycle.

It’s kind of like walking up a

mountain, then falling off a

cliff. The compressor char-

acteristic curve to the left

of the surge point creates

a total characteristic curve

that looks like a sine wave,

as seen in Figure 1. The pos-

itive slope immediately to

the left of the surge point

(maximum compressor

pressure rise) creates pos-

itive feedback that causes

the operating point to jump

from point B to point C, the

start of the negative slope.

The operating point walks

along the negative slope

from C to D, the point of

zero slope (minimum com-

pressor pressure rise), and

then jumps to the right back

to the starting point A. If a

surge valve is not opened,

the process repeats itself,

resulting in oscillations.

Note that the jumps in the

suction flow measurement

between peaks and valleys

are not seen in pressure

measurements due to the

smoothing by suction and

header volumes.

The jumps are highly dis-

ruptive and damaging

due to high axial thrust

and radial vibration. Surge

cycles damage bearings

and decrease efficiency

with each cycle. For axial

compressors, the damage

may be measureable after a

10.0

8.33

6.67

5.00

3.33

1.67

0.00

10.0

8.33

6.67

5.00

3.33

1.67

0.00

Callouts: [note that the outer oval loop is the “path,” the inner S-shaped

Positive slope creates positive feedback that causes flow to jump from B to C and D to A

Characteristic curve stops at zero slope

Curv

e, p

si

Path

, psi

BC

D21

A

-300 -187 -75 37 150 262 375 487 600acfm

SURGE AS SEEN AND UNSEENFigure 1: At point B, where the compressor characteristic curve slope is zero, the operating point jumps to point C. The precipi-tous drop in pressure signals the start of the surge cycle and flow reversal (negative ACFM). As the plenum volume is emptied, the operating point follows the curve from point C to point D, where the slope is again zero, and then jumps to point A.



few surge cycles. The total

number of surge cycles

provides a good metric of

the total loss in compressor

efficiency. It is imperative

to prevent the surge and

ensure sustained recovery.

SURGE SETPOINTThe surge controller set-

point should be offset to

the right of the surge curve

on the compressor map,

as shown in Figure 2. If the

surge setpoint follows the

shape of surge curve, the

offset can be optimized to

be on the longitudinal axis

of the efficiency ellipses.

The size of the offset

depends on the speed of

the automation system and

the tuning of the surge con-

troller. Some plants may

think of the surge curve as

being the point where the

surge valve opens. In this

case, integral action must

be greater than the pro-

portional action. However,

the extra integral action

causes a larger overshoot of

the surge setpoint, neces-

sitating the setpoint offset

to be increased accord-

ingly, which generally

corresponds to lower oper-

ating efficiency.

Most other plants see the

surge setpoint as being

the best operating point,

when surge valves are open

through tuning so that pro-

portional action dominates

integral action, preventing

overshoot. Using higher

controller gain rather than

a lower reset time gives a

faster correction. Whether

an automation system

can achieve this depends

on attention to the surge

valve’s 86% response time,

transmitter update rate

and damping setting, and

controller scan time and

execution rate. In general,

the summation of the valve

response time, transmitter

damping and ½ of each

update rate, scan time and

execution rate must be less

than 2 seconds. For closer

operation to the surge

curve and to reduce dire

consequences from surge,

the total must be less than

[if you can label all three lines with one “speed”, then just 60%, 80%, 100%]

Pres

sure

rise

, psi

Long

axis Efficiency

ellipses80%60%

Surg

e cur

ve

Contr

oller s

etpoin

t 100% speed

80% speed

60% speed

Inlet flow, acfm

EFFICIENT EVASIONFigure 2: The optimum surge setpoint follows the shape of the surge curve with an offset to intersect longitudinal efficiency el-lipses. The offset is large enough to prevent surge.



1 second. How fast the automation system

really needs to be and the required tuning

of the surge controller is best determined

by running a first-principle dynamic model

that includes a momentum balance as well

as material and energy balances. A word

of caution here is that some I/O scan rates

may be much slower than the fastest con-

troller execution rate.

SURGE CONTROL SYSTEMEven a fast feedback controller is unable

to get a compressor out of severe surge

because of the huge jumps in flow. What’s

needed is an open-loop backup that forces

the surge valves to immediately open,

and holds them open for sufficient time

to sustain operating point stability before

allowing the feedback controller to start

to close the surge valves. The open-loop

backup is triggered by a large predicted

overshoot of the surge setpoint to prevent

surge, or a precipitous drop in flow indicat-

ing an actual surge.

An innovation uses a predicted overshoot

via a fast future value that’s generated by

the rate of change of a decreasing flow,

with a good signal-to-noise ratio multiplied

by the total loop dead time, with updates

every controller execution. The open-loop

backup simply puts the feedback controller

into a remote output mode that is seen by

operator. The remote output is immedi-

ately stepped up to a position that typically

prevents surge, but is incremented every

execution until the future value stabilizes,

putting the surge controller bumplessly

back in cascade with the surge setpoint

computed to sustain an offset from the

surge curve. Many suppliers of standalone

compressor controllers have proprietary

control strategies providing feedback

control, with a backup requiring special

expertise and tuning.

External-reset feedback (ERF), also known

as dynamic reset limit, in the surge con-

troller, with a fast readback of actual valve

position, enables up and down setpoint rate

limits in the analog output blocks to provide

fast opening and slow closing of the surge

valves without the need to retune the surge

controller. ERF can also eliminate oscilla-

tions from valve resolution and sensitivity

limits (as seen in the Control feature article,

“How to specify valves and positioners that

don’t compromise control,” March ’16, p.

39, www.controlglobal.com/articles/2016/

how-to-specify-valves-and-positioners-that-

dont-compromise-control).

The surge control system principles are

basically the same for surge vent valves

and surge recycle valves. At least two

valves in parallel are used to provide

redundancy, particularly since surge valves

might not open after sustained operation

in closed position, where stiction from seal

or seat friction is greatest. For multiple

stages, there are generally recycle surge

valves and a compressor surge control



system for each stage. Ratio dividing may

be used to proportion the different pres-

sure rises for each stage.

Figure 3 shows the surge control system

with recycle surge valves and two down-

stream users feeding reactors. Many

systems have more reactors. A valve posi-

tion controller can minimize the pressure

setpoint to increase compressor efficiency

by pushing a user valve to the maximum

effective throttle position (e.g., 60%). The

valve position controller has external reset

feedback with setpoint rate limits on the

compressor pressure controller to provide

a gradual, smooth optimization with a fast

getaway for a disruption.

Not shown in Figure 3 is feedforward action

to deal with the fast closing of user valves.

What may seem to be a slow enough clos-

ing of a user valve can be troublesome

because of the quick opening character-

istic of on-off valves triggered by safety

Suction

RecycleW

BY1-1

ZC1-4

ZY1-4

DY1-1

SC1-2

FC2-2

FC2-1

FT2-2

PC1-3

FC1-1

ST1-2

FT1-1

FY1-1

DPT1-1

PY1-1a

PY1-1b

PT1-3

FT2-1

Openloop

backup ROUT

Maximumfeed valveposition

Derivativeof flow

Steam

Driver CompressorDischarge

Hi signalselector

Reactor 2feed

Reactor 1feed

Furthestopen valve

positionSP

SP

SP

SP

OPEN LOOP BACKUP AND OPTIMIZATIONFigure 3: The surge flow controller FC1-1 and enhanced PID valve position controller ZC1-4 have ex-ternal reset feedback for directional move suppression for fast opening with slow closing of surge valves for fast getaway in abnormal situations and gradual optimization. The derivative of the suction flow computed by DY1-1 uses a deadtime block to provide immediate updates. This derivative can be multiplied by the deadtime and added to the current suction flow to predict flow one deadtime into the future. Each reactor feed valve position can be used to provide a feedforward signal.



instrumented systems, where nearly all of

the flow change occurs within 10% of closed

position. The feedback and feedforward

signals must be linearized based on the

installed flow characteristics of the surge

valves and user valves, respectively. If a

high-rangeability, fast and reliable user flow

measurement is used for the feedforward,

the characterization of the feedforward

signal for the feedforward summer is unnec-

essary. Much more detail, with a focus on

practical essentials, is offered in the book

Centrifugal and Axial Compressor Con-

trol (www.momentumpress.net/books/

centrifugal-and-axial-compressor-control).

In Figure 3, the derivative of the suction

flow computed by DY1-1 uses a deadtime

block to provide immediate updates with

good signal-to-noise ratio. This derivative

can be multiplied by the deadtime and

added to the current suction flow to give

a predicted flow one deadtime into the

future. This plus reactor feed valve position

can be used to provide a feedforward signal

to help surge control deal with user (i.e.

reactor) shutdown.

The same type of calculation used to

give a future value can be used to find

and document surge points on the surge

curve on the compressor map. Detecting

a nearly zero rate of change in pressure

for a change in suction flow indicates a

surge point. Detecting a severe rate of

change of suction flow indicates a surge

cycle, with extreme negative rate of

change signifying the beginning and an

extreme positive rate of change signify-

ing the end of each surge cycle. Future

values can be computed with a good

signal-to-noise ratio and preemptive cor-

rections (as noted in the Control Talk blog,

“Future PV values are the future,” www.

controlglobal.com/blogs/controltalkblog/

future-pv-values-are-the-future).

COMPRESSOR MODELDeeper understanding of compressor surge

control, the dynamic requirements of the

automation system, setpoint optimization,

surge control system design with future

values, and surge curve identification is

best achieved by using a virtual plant

(digital twin) with a complete compressor

model. This digital twin can generate all

of the plots in this article and give much

more. This knowledge is nearly impossi-

ble to obtain elsewhere, and is essential

for preventing compressor damage, loss

of compressor efficiency, shutdowns and

hazardous operation of downstream exo-

thermic reactors.

The compressor models seen in indus-

try to date critically lack the momentum

balance needed to show the path, and

to include the normally unseen compres-

sor characteristic curve to the left of the

surge point. The momentum balance inte-

grated into the digital twin, which is the

basis of this article, is developed from



innovative research documented by E. M.

Greitzer in “The Stability of Pumping Sys-

tems—The 1980 Freeman Scholar Lecture”

(Journal of Fluids Engineering, June ’81,

p. 193–242) and subsequently confirmed

through testing by K.E. Hansen et al. in

“Experimental and Theoretical Study of

Surge in a Small Centrifugal Compressor”

(Journal of Fluids Engineering, September

1981, p. 391–395).

The surge model results and automation

system requirements are also described in

the previously mentioned book, Centrifugal

and Axial Compressor Control. Additional

guidance and a video demonstration are

at www.controlglobal.com/articles/2018/

compressor-control-resources.

Invest some of your own time to see the

future of a synergy between modeling and

control. Use the model to learn what’s truly

important and what’s really needed. Don’t

take a back seat, but instead, seek to pro-

vide the leadership to show what you and

our profession can do to make plants safer

and more productive.

Greg McMillan is a Control columnist, Hall of Fame

member and ISA Lifetime Achievement Award recipient.

Chris Stuart, software engineer, R&D/Engineering and

Thomas Hildebrand, simulation engineer 1, Systems/

Project Engineering at Emerson Automation Solutions

can be reached at [email protected] and

[email protected].



flow measurement - control globalflow measurement. fall, 2019. ehandbook. table of contents. how to...

Documents