Quick Check Pump Performance Using Motor Data and Field Measure Current

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There is a centrifugal pump working on the field and a flow meter is continuously measuring the flow rate delivered by the centrifugal pump. You noticed that flow meter indicating flow rate same as normal recorded flow rate, however, the downstream system indicating that there is reduction in flow. Flow meter Calibration is one of the normal way to confirm if the flow meter is working correctly. Prior to this, you may consider the following approach to quickly check if the pump delivering good flow and cross check with the flow meter.

This approach basically use the field tested pump curve as follow.

Field tested pump curve

Pump power consumed by pump shaft,

Es = (dH x Q x SG) / (3960 x Pump Eff. x Kv) [Eq. 1]

where,
Es = Pump shaft power (HP) consumed
dH = Pump head (ft)
SG = fluid specific gravity
Pump Eff. = Pump efficiency
Kv = Viscosity Correction Factor

Power deliver by a motor to pump shaft,

Em = (1.732 x V x I x Motor Eff. x P.F.) / (746) [Eq. 2]

where,
Em = Power deliver by motor to pump shaft
V = Voltage (v)
I = Current (amp)
Motor eff. = Motor efficiency
P.F. = Motor Power Factor

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Case study
A centrifugal pump transferring water from tank to a drum. A pressure transmitter located on the pump discharge. This estimated pump head based on differential pressure is 2000 ft. Fluid SG is 0.9932. From field, Voltage and current for the centrifugal pump are 460 volts and 323.1 amp. The flow meter is indicating flow rate of 385 gpm. Check if the flow meter is correctly measuring the flow rate.

From motor catalog, you may obtain motor efficiency (motor Eff.) and power factor (P.F.). For example, a motor with Motor Eff = 95% and P.F.=90%. The motor efficiency and P.F. may varies a bit (2%-5%). But they can be assumed same.

Power consumed / delivered,
From [Eq. 2],
==> Em = (1.732 x V x I x Motor Eff. x P.F.) / (746)
==> Em = (1.732 x 460 x 323.1 x 0.95 x 0.9) / (746)
==> Em = 295 HP

From above pump curve,
With Em = 295 HP
==> Flow, Q = 400 gpm, Pump Eff. 68% and Pump Head = 2000 ft.

Q = 400 gpm > 385 gpm as measured by flow meter. This indicates that the flow meter may not perform correctly.

Cross check with [Eq. 1],
As fluid is water,
==> Kv = 1.

For other type of fluid, may check out the viscosity correction factor using curve (by HI) presented in "Quick Check if Pump Performance Curve (Water) is Good for High Viscosity Fluid".

From [Eq. 1],
==> Es = (dH x Q x SG) / (3960 x Pump Eff. x Kv)
==> Q= Es x (3960 x Pump Eff. x Kv) / (dH x SG)
==> Q= 295 x (3960 x 0.68 x 1) / (2000 x 0.9932)
==> Q = 400 gpm


It is useful to generate a Current versus Flow curve as follow :

Current versus Flow curve

With this curve, operator may use it quickly check against the flow meter.

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FAYF - FLOWMETER SELECTION
by Rebekkah Marshall
Briefly discusses general selection criteria, flowmeter accuracy and turndown...
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Following are 6 articles in series related to Piping for Process Plants. by W. M. Huitt, W. M. Huitt Co.

Piping for Process Plants Part 1: The Basics

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Typical of the topics that will be covered in this series are the following:

* With respect to ASME flange ratings — Is the correct terminology 150- and 300-pound flange, or is it Class 150 and Class 300 flange? And do the 150 and 300 actually mean anything, or are they simply identifiers? Similarly, with respect to forged fittings, is the terminology 2,000-pound and 3,000-pound, or is it Class 2000 and Class 3000?

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Piping Design, Part 2 - Flanges

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Flow Element (FE) Upstream or Downstream of Control Valve (CV) ?

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A question raised by a young engineer.

Should a Flow element (FE) be located upstream or downstream of a control valve (CV) ?

There is no fix rules governing the location of this flow element. It is very much subject to few factors i.e. fluid condition of the measured fluid, properties fluctuation, etc.

a) Fluid Characteristic

A flow element is general prefer single vapor or liquid phase as compare to two phases gas-liquid (2Ph-GL) flow. First to check is the fluid characteristic downstream of control valve. For example, liquid control valve (LCV) maintaining level of a separator. The fluid feeding the LCV is probably a liquid at bubble point. Due to static head it potentially maintain as liquid from separator outlet nozzle upto LCV. However, there is pressure drop across the LCV and 2Ph-GL flow is likely to occur downstream of LCV. Under this condition, it is always prefer to locate flow element upstream of LCV.

Let take another example for vapor flow line. A pressure control valve (PCV) maintaining a constant saturated gas flow to downstream system i.e. fuel gas system from a separator. In the event there is large pressure drop across the PCV, Joule-Thompson (JT) effect results significant liquid formation downstream of PCV. Similarly, it is always prefer to locate flow element upstream of PCV.

b) Pressure & Temperature Fluctuation

Large pressure and temperature fluctuation would probably leads to properties (i.e density) changes. In general, a vapor system would experience severe changes as compare to liquid system. Thus, large pressure and temperature fluctuation may results a large deviation in properties and lead to difficulties in select a good flow element for the service. It is always prefer to locate flow element in the stream with "less" properties change .

For example, a slug catcher is having a pressure control valve (PCV) feeding gas to booster compression system. The booster compression is targeted to maintain a constant pressure of

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the suction scrubber. As slugcatcher pressure (upstream of PCV) may fluctuate due to slugging and pigging operation whilst the pressure (downstream of PCV) at compressor suction scrubber is rather constant, it is always prefer to locate flow element downstream of PCV. It shall be noted that there is potential liquid formation downstream of PCV which prefer to locate PCV upstream of PCV. Both rule-of-thumbs are contradicting between each and other, a process engineer shall weight both impacts to locate it at right location.

c) Pressure Rating

High pressure rating of flow element will be costly. Thus, it is always prefer to locate the flow element at the system with lower pressure rating, downstream of control valve.

d) Vibration

Pressure drop across control valve would partially convert its energy to noise and vibration. The vibration (higher intensity) would travel along fluid flow direction. Some flow element (i.e. Coriolis,Vortex) is sensitive to vibration. Vibration sensitive flow element is always advisable to locate upstream of control valve.

A process engineer may consider above factors to locate the flow element. Apart, process engineer is always advisable to verify this subject with instrument engineer and vendor for proper location and correct flow meter type.

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• Use Ultra-Sonic Flowmeter in FLARE Gas Header for emission monitoring
• FAYF - Flowmeter selection in brief...
• Useful Documents Related to Control Valve
• Restrcition Orifice Used in Many Applications in Different Manners
• A refresh to Process Engineer on few phenomenons in restriction orifice
• Why Restriction Orifice is some distance from Blowdown valve ?

Use Ultra-Sonic Flowmeter in FLARE Gas Header for emission monitoring

Increasing greenhouse effect and global warming has resulted environment changed dramatically. Signing of Kyoto protocol is part of the cumulative effort to minimize global warming. Rules and regulations becoming stricter and demands for continuous flare gas measurement and recording has become one of the mandatory requirements.

“Within the United States, the Environment Protection Agency (EPA, www.epa.gov) has enforced emission levels of NOx (Nitrous Oxides) and highly reactive volatile organic compounds (HRVOC) through individual states with allocations and fines. In Texas, California, and New Jersey the ozone depletion has led to stricter total emissions and monitoring of HRVOCs.. In Texas, the Texas Commission of Environmental Quality (TCEQ, www.tceq.state.tx.us) Chapter 115 Regulation has set the levels for each flare stack. The new regulations require a flare gas flowmeter to be specified to have +/- 5 percent inaccuracy at 30, 60, and 90 percent of range under its installed condition”

Calculating flare gas has gone into history. One of the biggest challenges of measuring flare gas is large TURNDOWN. Flare gas flow can ranged from low fuel gas purge during normal operation to large flow during emergency relief and/or total plant blowdown.

Common flowmeter type such as differential-pressure, vortex-shedding, and insertion thermal mass meters, etc are unable to meet such low turndown requirement. Ultra-sonic flowmeter is generally used in flare gas measurement & recording. See some features and benefits of an Ultra-Sonic Flowmeter HERE and HERE

Apart from large turndown capability (from 0.03 m/s to 85 m/s), ULTRA-SONIC flowmeter also has the following advantages in flare gas application :

- Extremely low pressure drop (virtual zero pressure drop)
- No internal, insertion and moving parts which potential create partial blockage of flare line
- Tolerate some condensed liquid
- Can take higher operating temperature (upto 260 degC)
- No affected by gas composition
- No maintenance

Thus, stringent authority requirements on flare gas measurement, ULTRA-SONIC flowmeter would be one of the best offers.

Further reading

• FLARE combustion efficiency
• The Future of Flare Gas Measurement
• Industrial Flares, Section 13.5. Sept. 1991
• How much do you flare? How to measure flowrates of flare gas accurately and reliably.
• Regulation 12, Rule 11: Flare Monitoring at Petroleum Refineries
• Point Source Monitoring to Identify and Reduce HRVOC Emissions

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CE - Evaluation and Advances in Flowmeters

CHEMICAL ENGINEERING magezine also shared the following articles within the community...If you are still fresh in Chemical and Process Industrial, I would recommend you read the first article by Jenniffer. It is simple and practical...Process / Plant expert like some of you...please proceed to second article. There are more consolidatred and philosophical / conceptual kind of approaches...

<<Evaluating Industrial Flowmeters>>

Jennifer Keith

Flow Technology, Inc

This article provides an overview of flowmeter selection criteria and practical advice for evaluating common meter designs and for pairing the right instrument technology with various flow-measurement applications is offered. Intangible factors shall also be considered which include familiarity of plant personnel, their experience with calibration and maintenance, spare parts availability, and mean time between failures at the particular installation site.

<<Advances in Industrial Flowmetering>>

Gernot Engstler

Endress + Hauser

High demand from chemical process industry (CPI) has pushed the manufacturer to provide high performance instrumentaion and flowmetering solutions include tools for the entire life cycle of a flowmeter.

From engineering, commissioning and start up to operations and maintenance, advanced diagnostics and onsite device-verification features help to maximize plant availability, instrument reliability and cost savings. Flowmeter instrumentation can even provide process status and analysis information that can be used to increase product quality and process efficiency. In addition to requiring high performance, users increasingly look for sensors that are easy to install, commission, use and maintain. Flowmeters that are tailored to the specific requirements of an industry or application can reduce the complexity and costs of operation. In this article, we look at some of the advanced functions of flowmeters.

Further Reading

• FAYF - Flowmeter selection in brief
• Everyone Wants an Insertion Magmeter

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FAYF - Flowmeter selection in brief...

CHEMICAL ENGINEERING magazine shared the following useful information....

<<

FLOWMETER SELECTION >>, by Rebekkah Marshall

Briefly discusses general selection criteria, flowmeter accuracy and turndown...

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