Inner UnitHousing

StaticLiquid

Blade

Path ofBlades

FlowingLiquid

Cam MeasuringChamber

MeterHousing

BladeBearing

Rotor

into the meter. The measuring element is less likely to beaffected by these stresses in the double-case meter.

Measuring ElementPD meters measure flow directly by separating the flowstream into discrete volumetric segments (measuringchambers). By totalizing these segments, it is possible todetermine the quantity passed through the meter.The measuring element also serves as a hydraulic mo-tor, absorbing energy from the flow stream to producetorque to overcome internal friction and to drive accesso-ries in the mechanical flow information output.Figure 1 illustrates some of the most common PD metermeasuring elements.

Technical Paper

Smith MeterTM PD Meter

Displacement Meters forLiquid Measurement

Bi-Rotor Meter

Rotating Paddle Meter

Oval GearMeter

Figure 1 — PD Meter Measuring Elements

Introduction

The purpose of this paper is to examine the positivedisplacement (PD) meter. The emphasis will be on thefactors influencing the design and performance of themeter for liquid petroleum measurement. However, thesefactors can be applied to other liquids as well.

History

PD meters have existed for over a century. Many of thedesigns were developed from either pumps or compres-sors. By the late 1930’s, PD meters were being usedextensively for custody transfer measurement of petro-leum liquids on tank trucks, loading terminals, and pipe-lines. By the 1960’s, PD meters had been developed thatcould handle flow rates in excess of 12,000 barrels perhour for large pipeline and ship-loading facilities. Mostwill agree that even today there is not a more accuratemeans of petroleum measurement available than the PDmeter.

Design and Construction

PD meters can be broken into three basic groups ofcomponents: the housing, the measuring element, andthe flow information output (mechanical or electronic).

HousingThe housing consists of inlet and outlet connections andserves as the pressure vessel containing the measuringelement. The connections can range in size from 3/4inch up to 16 inch and can be ANSI or DIN flange, NPT,RTJ, or Victaulic. The working pressure, depending uponthe construction, can be up to 1,480 psi (ANSI Class600). Higher pressures are also available. The maximumflow rate varies with the connection size up to 12,500BPH.PD meters can be either of single- or double-caseconstruction. In the single-case construction, the exter-nal housing serves both as a pressure vessel and as themeasuring element housing; whereas, with double-caseconstruction, the measuring element is surrounded bythe separate external housing. This offers the advantageof eliminating the pressure differential across the mea-suring element. In some cases, pressure variations assmall as 20 psi can significantly affect the accuracy of asingle-case meter.Another advantage of the double-case meter is that themeasuring element can be easily removed for hydrotestand line flush on start-up or for service.Sometimes, line connections can impart significant stress

Double-Case Rotary Vane Meter

SlidingVane Meter

Issue/Rev. 0.2 (10/98) Bulletin TP01007

The Most Trusted Name In Measurement

Issue/Rev. 0.2 (10/98)Page 2 • TP01007

CCW

Calibrator

CalibratorDrive Gear

CCW

CW

CW

CCW

Idler Pinion

JackshaftPinion

Jackshaftand GearPacking

Gland

RotorGear

Figure 2 — Accessory Drive

In addition to being able to accurately measure liquidthroughput, a high performance measuring element willhave the following design features:a. Low Pressure Drop - minimizes pump size and oper-

ating costs and also helps to produce accurate mea-surement.

b. Low Mechanical Friction - improves the service lifeand helps to produce accurate measurement.

c. High Driving Torque - especially important when themeters mechanical flow information output is fittedwith a “stack” of accessories.

d. Non-Jamming Rotation - allows handling of largeforeign particles in large pipeline applications wheresudden stops can cause damaging pressure surges.

Flow Information OutputPD meters normally have a mechanical flow informationoutput that is fitted with either mechanical counters orpulse transmitters. Mechanical counters directly registerthe volume while pulse transmitters must be connectedto electronic volume registration instrumentation. In ei-ther case, a gear train is used to convert the somewhatarbitrary displacement volume of the measuring elementinto a convenient volume-per-revolution of the accessorydrive. For example, a two-inch Smith MeterTM Rotary VanePD Meter has a nominal displacement of 0.364 gallons forevery revolution of the rotor. The gear train will convert thisinto a more convenient nominal one revolution of the ac-cessory drive for every five gallons passing through themeter.Since the gear train begins with the rotation of the measur-ing element (inside the meter) and ends with the acces-sory drive (outside the meter), a packing gland is re-quired on the shaft which penetrates the housing (seeFigure 2). The gear train normally reduces the speed ofthe shaft at this point which reduces the torque gener-ated by the measuring element to overcome this highfriction point. The packing gland is subject to wear and,therefore, requires periodic service.Some meters use a magnetic coupling instead of a pack-ing gland to transfer the drive through the housing, therebyeliminating the friction and solving the wear problem.

a. Mechanical CountersWhen mechanical counters are used to indicate thetrue volume throughput, the meter is generally fittedwith an adjuster or calibrator. This device is used tomake fine adjustments compensating for manufactur-ing variations and liquid properties. The Smith MeterTM

PD Meter uses a double overriding, clutch-type calibra-tor. This device is variably adjustable to increase therevolutions coming into it by up to 10%. Therefore, thegear train must under-register the throughput so thecalibrator can add the correct (adjustable) amount toprovide true volume indication. Ninety-six percent gear-ing is common in a Smith MeterTM PD Meter. In otherwords, the input to the calibrator in the 2-inch metermentioned above would be a nominal 0.96 revolutionsfor every five gallons passing through (96%, 5:1 gallongearing). The calibrator is then adjusted under operatingconditions to produce exactly one revolution for everyfive gallons passing through the meter. Other gearings,such as 1:1 gallon, 1:1 dekalitre, 1:1 barrel, are avail-

able as the application requires.Sometimes nominal 100% gearing is used and the “live”calibrator is replaced with a “dummy” calibrator. Thedummy has a spacer shaft that drives the accessoriesand is not adjustable. When it is used, it is necessary tomultiply the registration by a meter factor in order toobtain true volume throughput.

b. Electrical PulsersIt is very common for PD meters to be fitted with pulsetransmitters. In this case, the throughput informationcoming from the measuring element is converted intoa precise ratio of pulses per volume. Normally, in thiscase, there is no adjustor or calibrator used since theelectronic instrument is programmed to make the ad-justment to true volume.There are PD meters available now that have no me-chanical output. They produce pulses directly fromthe rotation of their rotor. The Smith MeterTM PRIMEMeter and the Brooks P Style meters are example ofthis technology. The advantage is no packing glandand non-cyclic pulse rhythms that allows for repeatableconsecutive proving runs when proved with a smallvolume prover. (See figure 3)

Figure 3 — PRIME Meter Section

➛

Direct Pulse Pickup

Smith MeterTM

PRIME 4 Meter

Issue/Rev. 0.2 (10/98) TP01007 • Page 3

Figure 6 — Effect of Viscosity on the Accuracy Curve

Flow Rate

% S

lippa

ge

100%

ZeroSlippage

16 cP8 cP

4 cP

2 cP

Flow Rate

% S

lipp

age

Sum of theOther Curves

Slippage Due To:

1. Hydraulic P

2. Mechanical P

∇∇

100%

Figure 4 — Accuracy Curve

P

∇

L c

P

∇

Slippage ( q ) =Xc

L cµ

X c

K

= Units Constant= Clearance Width= Clearance Length= Pressure Drop Across the Clearance= Absolute Viscosity of the Liquid

KXL

cc∇

Where:

q

µP

3

differential pressures and the corresponding slippageexplain the reason for the accuracy of a PD metervarying with the flow rate.Slippage through the clearances is characterized bythe equation shown in Figure 5 and is affected by thefollowing factors:a. Flow Rate

As shown in Figure 4, the percentage of slippagechanges with flow rate due to the varying differen-tial pressure produced by hydraulic and mechani-cal friction.

b. ViscosityAs the viscosity of a liquid increases, it is moredifficult for it to pass through the clearances of themeasuring element.Figure 6 shows how the accuracy curve of a PDmeter is affected by the viscosity of the liquid beingmetered. Note that doubling the viscosity results inhalving the percentage of slippage. At viscositiesgreater than about 16 cP, the amount of slippage

Figure 5 — Slippage Through PD Meter Clearances

Accuracy Theory

The factors affecting the accuracy of the PD meter can bedivided into two groups; those that affect the displacementof the measuring element and those that affect the amountof slippage bypassing the measuring element.

1. DisplacementThe displacement of a PD meter is determined by thesize of the volumetric segments formed in the meas-uring element. Factors influencing the physical andapparent size of these segments will affect the accu-racy of the meter:a. Temperature

Increasing the temperature increases the displace-ment of the meter because of thermal growth. Thiscan be related to the cubical expansion coefficientof the material forming the segments. In the stan-dard Smith MeterTM Rotary Vane PD Meter, the alu-minum blades grow faster than the other cast ironparts. This causes the effective displacement toincrease because the blades “sweep” a greater vol-ume. The combined effect is typically about 0.02%for a 10°F change in fluid temperature.

b. WearWear has the effect of increasing the displacement.In the Smith MeterTM Rotary Vane Meter, as the camor blade bearings wear, the blade is allowed to moveoutward, sweeping a greater volume. Wear is nor-mally slow and predictable.

c. ViscosityThe viscosity of liquids causes a film to cling to thesurfaces of the measuring element. As the film thick-ness increases, the displacement is reduced to thepoint where the film can be no thicker because ofthe wiping action of the parts. Further increases inviscosity have no effect on the displacement.

d. CoatingsLike the film created by the viscosity of the liquid,coatings or deposits can build up and reduce thedisplacement of the measuring element. If the coat-ing thickness remains constant, there is normallyno problem. However, the thickness of the coatingcan vary dramatically in some crude oils containingparaffin which has a melting point near the operat-ing temperature. Formation or melting can occurwith slight changes in temperature andcan significantly change the displacement.

2. SlippagePD meters have clearances between moving and sta-tionary parts in the measuring element. These aresometimes referred to as capillary seals. Differentialpressure across these parts will cause a flow that isnot accounted for in the displacement. This flow iscommonly referred to as “slippage.” Figure 4 showsslippage as a percentage of flow through the meter.There are two causes of differential pressure across ameter: hydraulically-induced because of flow and me-chanically-induced because of internal friction and ex-ternal accessory driving torque. The net result of these

Issue/Rev. 0.2 (10/98)Page 4 • TP01007

nears zero and the percentage change in slippage atvarious flow rates is negligible.

c. FrictionWhen mechanical friction increases, more pres-sure differential is required across the measuringelement. Figure 7 shows how the accuracy curve isaffected by high friction. If the percentage of slip-page is not lower at 50% of flow rate than at 100%,there is probably abnormally high mechanical fric-tion. It could be because internal parts have wornand are dragging or it could be due to excessiveaccessory loads.

d. WearAs parts wear, the clearances in the measuring ele-ment will change. Small variation in the clearancescan cause significant changes in the amount of slip-page (see Figure 8). This is due to the cubical relationof the clearance to the slippage (Slippage Equation).Loss of rotor end clearance in the horizontal SmithMeterTM Rotary Vane Meter will cause the compoundingeffect of increasing the slippage through the upper rotorend clearances and increasing the slippage through allclearances due to the high friction load.A good measuring element must have very small clear-ances between the moving parts in order to maintainlow slippage. However, with small clearance there islittle room for wear before contact will cause high fric-tion.

Flow Rate

% S

lipp

age

100%

Normal

High

Very High

Figure 8 — Effects of Clearances on Accuracy Curve

Flow Rate

% S

lipp

age

100%

ZeroSlippage

0.002 inches

0.004 inches

1

8

Application Considerations

When measuring a flow stream with a PD meter is desired,there are several factors which must be considered:

1. Flow RateLine size is an important factor in sizing a meter; how-ever, flow rate and flow range are usually the decidingcriteria. In high flow rate situations, multiple parallelmeter runs are normally more economical since theprover (normally a permanent part of the system) canbe much smaller. Another advantage of multiple meterruns is the ability to isolate one meter for servicingwhile diverting the flow stream to the other meters.Generally, it is advisable to operate the meter at re-duced flow rates. This will have the effect of extendingthe life of the meter. The life of a PD meter is consid-ered to be inversely proportional to the square of theflow rate. Therefore, operating a meter at 80% of maxi-mum will extend its life by about 50%.

2. PressureThe maximum working pressure rating of the metershould always be higher than the maximum pressureof the application. Thermal relief must be considered ifthe meter can be isolated between valves.

3. TemperatureIt may be necessary to alter the construction of themeter to suit the operating temperature. If the tempera-ture is very low (less the -20°F), it may be necessaryto consider low temperature steel for the housing. Asthe operating temperature increases, it may be neces-sary to apply special clearances that anticipate ther-mal growth of parts in the measuring element.

4. ViscosityWhen the viscosity of the liquid being metered is verylow, the lubricity is also very low. Special constructionutilizing low friction bearings is recommended on LPGand lighter liquids.

Most PD meters can handle viscosities up to 400 cP.Higher viscosities may require increased clearancesin the measuring element or derating the maximumflow rate of the meter.

5. StrainersIt is normally advisable to protect the meter with astrainer. The Smith MeterTM Rotary Vane PD Meter canpass fairly large particles without damage. A four-meshstrainer will normally provide adequate protection forthe meter. In large pipeline applications, this is impor-tant since it helps keep the pressure drop low.

6. Material CompatibilityCare must be taken with the compatibility of the meter’smaterial of construction and the liquid to be metered.Different seals and metals are usually available tohandle most petroleum applications.

Conclusion

Figure 7 — Effects of Mechanical Friction on theAccuracy Curve

Issue/Rev. 0.2 (10/98) TP01007 • Page 5

Vis

cosi

ty,

(

cP)

Flow Rate, Q (gpm)

3 10 30 100 300 1,000 3,000 10,000 30,000 100,0000.1

0.1

0.3

1

3

10

30

100

>100

PD Best

TurbineConsidered

PD ConsideredTurbine Best

<

µ

Figure 9 — PD and Turbine Meter Selection Guide

The petroleum industry demands precise measurementfor custody transfer. While there are other meteringtechnologies available, the PD Meter continues to be themeter by which all other meters are compared. Figure 9,from the API Manual of Measurement Standards showsthat PD meters excel when the application is on higherviscosity oils. While many are using turbine meters toreplaced PD meters, the PD meter continues to be thepreferred meter when accurate and dependable meas-urement is imperative.

Acknowledgment

This paper was originally presented at the InternationalSchool of Hydrocarbon Measurement (ISHM), Universityof Oklahoma, May 1997.

Printed in U.S.A. © 10/98 FMC Measurement Solutions. All rights reserved. TP01007 Issue/Rev. 0.2 (10/98)

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