Positive Displacement Flow meter working principle

Positive Displacement flow meters are the only flow measuring technology to directly measure the volume of fluid that passes though the flow meter. It achieves this by trapping pockets of fluid between rotating components housed within a high precision chamber. This can be compared to repeatedly filling a beaker with fluid and pouring the contents downstream while counting the number of times the beaker is filled.

Rotor rotational velocity is directly proportional to flow rate, since the flow of fluid is causing the rotation.  electronic flow meters the rotating components contain magnets that activate various sensor options located outside the fluid chamber. Mechanical positive displacement flow sensors rely on the rotation to drive either a magnetic coupling or a direct gear train connected to the mechanical counter.

PD flow meters do not require a power supply for their operation and do not require straight upstream and downstream pipe runs for their installation. Positive displacement flow meters are available in sizes from in to 12 in and can operate with turn downs as high as 100:1, although ranges of 15:1 or lower are much more common. Slippage between the flow meter components is reduced and metering accuracy is therefore increased as the viscosity of the process fluid increases.

The process fluid must be clean. Particles greater than 100 microns in size must be removed by filtering. Positive displacement meters operate with small clearances between their precision-machined parts; wear rapidly destroys their accuracy. For this reason, PD meters are generally not recommended for measuring slurries or abrasive fluids. In clean fluid services, however, their precision and wide rangeability make them ideal for custody transfer and batch charging. They are most widely used as household water meters. Millions of such units are produced annually. In industrial and petrochemical applications, Positive displacement flow sensors are commonly used for batch charging of both liquids and gases.

Although slippage through the positive displacement flow meter decreases (that is, accuracy increases) as fluid viscosity increases, pressure drop through the meter also rises. Consequently, the maximum (and minimum) flow capacity of the flow meter is decreased as viscosity increases. The higher the viscosity, the less slippage and the lower the measurable flow rate becomes. As viscosity decreases, the low flow

Are DP meters accurate?

One of the main benefits of using a PD flow meter is the high level of accuracy that they offer, the high precision of internal components means that clearances between sealing faces is kept to a minimum. The smaller these clearances are, relates to how high the accuracy will be. Only fluid that is able to bypass this seal does not get counted, this is known as ‘by-pass’ or ‘slippage’.

Another benefit is the flow meters ability to process a huge range of viscosities and it is not uncommon to experience higher levels of accuracy while processing high viscosity fluids, simply due to the reduction of by-pass. When considering and comparing flow meter accuracy, it is important to be aware of both ‘linearity’ i.e. the positive displacement flow meters ability to accurately measure over the complete turn down ratio, and ‘repeatability’, the ability to remain accurate over a number to cycles. This is another area where positive displacement flow meters excel, repeatability of 0.02% and 0.5% linearity are standard.


If the correct flow meter has been selected for an application it can be expected to perform without error for many years. Frequently PD flow meters are sent in for servicing and recalibration that have been in the field for 10 sometimes 20 years continuous use. This reliability is largely due to the fact that this same proven technology has been in use for over 60 years, allowing the major advances to be focused within the fields of tribology and achieving the required precision at a reasonable cost.

Low Maintenance

The level of recommended maintenance is heavily influenced by the application. For example if a flow meter is processing a fluid that displays lubricating properties i.e. oil, then routine maintenance can virtually be eliminated. If however the fluid has very poor lubricating properties then it is best to discuss the maintenance requirements with your distributor. It is very uncommon that any maintenance to a positive displacement flow meter would be more frequent than that of other equipment within the same system and can be scheduled to be carried out at the same time, minimalizing downtime.

Positive Displacement Flow meter application

The industries where they are used in descending order are oil and gas, water and wastewater, chemical, power, pharmaceutical, food and beverage, pulp and paper, metals and mining and aerospace. 

Design Pressure :-

Low-pressure designs available from 5 to 100 PSIG (0.34 to 6.9 bars); high-pressure units available up to 1440 PSIG (100 bars)

Design Temperature: –  

Standard units can be used from  − 30 to 140 ° F (–34 to 60 ° C)

Price :- Its depend on size from $ 150 to $ 5000.

Positive displacement flow meter advantages and disadvantages

Advantage :-

  • High level of accuracy (0.5% as standard) higher level of accuracy available upon request
  • Ability to process a large range of fluid viscosities <1 cP – >500000 cP
  • Low maintenance
  • High pressure capabilities
  • Electronic and mechanical versions available
  • Spare parts availability

Disadvantage :-

  • While positive displacement flow sensors are very robust, there are limitations to their usage. Firstly they should not be used for fluids that contain large particles, unless these can be filtered out prior to the fluid entering the measuring chamber.
  • They are also not suitable for applications where large pockets of air are present within the fluid.
  • Another factor that requires consideration is the pressure drop caused by the PD meter; although these are minimal they should also be allowed for in system calculations.

Positive Displacement flow meters technical specification 

Positive Displacement flow meters

Inspection and Test Plan | Mass Flow Meter ITP

This content provides you a sample mass flow meter inspection and test plan. This is a draft mass flow meter ITP and you need to modify it to meet your specifications.

Mass Flow Meter Inspection and Test Plan TaskVendorTPI InspectionClient
Mass Flow Meter Mill test reportsHRR
Mass Flow Meter Pressure testHWR
Mass Flow Meter Performance test including calibration and hysteresis testsHWR
Mass Flow Meter Insulation resistance testHWR
Mass Flow Meter Final visual / Dimension InspectionHHR
Mass Flow Meter Documentation review prior to release(Final activity of Technical Inspection)HHR
Mass Flow Meter Pre-shipment InspectionHHR

H: Hold Point implies that relevant production activities shall not proceed until the continuation of work is permitted by Purchaser. (Notification req’d)

W: Witness point implies that Purchaser intends to witness the designated inspection feature. If Purchaser decides not to witness the relevant feature, production can proceed without permission of Purchaser. (Notification req’d)

S: Witness, but spot check basis. Initial operation will be witness point and subsequent operation will be witnessed at discretion of Purchaser considering the results of previous inspection. (Notification not req’d : Randam Inspection)

R: Review inspection & test records (Notification not req’d)

General Note :

1) Mass Flow Meter Witness/hold point by Code Inspector, when applied by Code /Local Regulation, is to comply with its requirements and to specify in Vendor’s ITP.

2) Type of Inspection by Mass Flow Meter Purchaser may be changed according to equipment criticality and vendor QC evaluation at job stage.

3) When requested in the requisition, detailed Mass Flow Meter ITP shall be submitted by Vendor.

4) Monitoring/Observation of Mass Flow Meter Vendor shop daily routine works (i.e.Storage of materials, Adherence to approved procedures, Testing tool calibration check, workmanship,cleanliness and etc.) shall be done by Purchaser during inspection visit.

5) 100%  Mass Flow Meter inspection which is covered by the combination of witness / spot check / record review inspection & tests listed above and vendor’s original QC activity shall be confirmed by Purchaser’s inspector prior to shipment.

6) Unless otherwise specified in REQ., the Mass Flow Meter sampling Q’ty of ” S (Spot Witness Check) ” shall be as follows:

3 to 20→3Pcs(all if total 2 and less) , 21 to 50→5Pcs, 51 to 100→10Pcs,101 to 200→20Pcs, 201 to 300→30Pcs, 301 to 500→50Pcs

Minimum 25 pcs or more of the commodity shall be arranged for H/W/S inspection and test at one time inspector’s visit as much as practical.

7) Final Mass Flow Meter Visual Inspection/Documentation Review/Pre-Shipment Inspection may Pre be combined within one time visit.

Positive Displacement Flowmeter installation

  • Thoroughly flush the service line upstream of the meter to remove dirt and debris.
  • Remove meter spud thread protectors. Note: To protect the meter spud threads, store the meter with the thread protectors in place.
  • Set the meter in the line. Arrows on the side of the meter and above the outlet spud indicate the direction of flow.
  • Do not over-tighten connections; tighten only as required to seal. Do not use pipe sealant or Teflon tape on meter threads.
  • With upstream shutoff valve only: Open the shutoff valve slowly, to remove air from meter and service line. Open a consumer faucet slowly to allow entrapped air to escape. Close the customer Faucet.
  • With upstream and downstream shutoff valves installed:
  • To test the installation for leaks: Close the outlet (downstream) shutoff valve. Open the inlet (upstream) shutoff slowly until the meter is full of water.
  • Open the outlet (downstream) valve slowly until air is out of the meter and service line. Open a customer faucet slowly to allow entrapped air to escape. Close the customer faucet.
  • When installing meters equipped with electronic registers that require connections to external devices, pleaserefer to installation instructions or diagrams provided by Master Meter.

Positive Displacement Flow-Meter Manufacturers

Positive Displacement Flow Meter Manufacturers / VendorsCountryWebsitePhone No.
BROOK CROMPTON LARGE INDUSTRIAL MOTORSUKwww.brookcrompton.com+44 1484557200
DANIELUKwww.daniel.com+44 1786433400
EMERSONFRANCEwww.amm-marseille.com+33 491602263
EMERSONUKwww.frco.com+44 1162822822
FOXBOROGERMANYwww.foxboro-eckardt.com+39 6419931
GRUPPO ISOIL SPAITALYwww.isoli.com+39 267071969
OVAL CORPORATIONJAPANwww.oval.co.jp+81 333605121
PETROL INSTRUMENTS S.R.LITALYwww.petrolvalves.it+39 69201941
SIEMENSGERMANYwww.powergeneration.siemens.com+49 9131180
SMITH METERCANADAwww.smithmeter.com+1 848985000
SOLARTRON MOBREYUKwww.solartronmetrology.com+44 1243833333
YAMATAKEJAPANwww.yamatake.com+81 334862111

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