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The importance of DP cell calibration

It has been said that the flowmeter is the cash register of the oil and gas industry. With present day oil and gas prices, the uncertainty of flowmeters is becoming more critically important – be they for fiscal or custody transfer duty or not.

Imagine, for instance, a natural gas metering station; where the meter monitors product worth $24 million per day, an uncertainty of 0.01 per cent could equate to $876,000 per year. Metering uncertainty affects performance calculations and could have significant implications on company operations and investment decisions.

Calibration of DP cells at High Static Pressure

Accurate measurements are an essential part of process control, but no measurement is absolutely precise. Uncertainties come from the limited accuracy of the measuring instrumentation and the associated equipment reading the data.

Differential Pressure cells on fiscal metering stations usually operate with a very high ratio between the level of static pressure and differential pressure. Usually static pressure in a gas line is in the region of 60 bar, whereas the DP level will be typically in the order of millibars. Therefore the effect of static pressure on the operation of a DP cell can be very significant, particularly on fiscal applications where even a very low percentage inaccuracy is very costly.

Note that the use of the phrase ‘fiscal metering’ does not imply any particular type or quality of instrument, merely the meter’s duty. Fiscal in this context refers to the metering of fluids where governments have a pecuniary interest, through royalties, or local taxation regimes.

With orifice plate flowmeters, the DP cell is the primary fiscal measurement in the determination of the volume of gas flowing in the pipeline, and it is extremely important that it gives an accurate value to the flow computer.

The only secure way to give this accuracy is by calibrating the DP cell in the same environment in which it carries out its duty, that is, at a relatively high static line pressure. Therefore it is essential that these cells are calibrated by a device which can give not just differential pressure, but differential pressure on top of a true static pressure. This is generally done with a twin-post dead weight tester, such as the DH-Budenberg unit shown (above right).

This unit may be operated on any gas, including methane and natural gas. The uncertainty levels on these devices, on differential pressure, are as low as 0.005 per cent of reading.

As the differential pressure is generated by the addition of extra weights, the resolution can be extremely small.

Standard Deadweight Testers, for gauge calibration, can never start from zero pressure with a small resolution, as the piston and weight carrier always have a mass.

With some of today’s Process DP transmitter manufacturers offering 0.025 per cent accuracy, it is easy to understand why such accuracy in the calibration equipment is necessary.

One major differential transmitter supplier quotes accuracies of 0.075 per cent if calibrated at working pressure but 0.125 per cent if the same transmitter is calibrated at atmosphere and then used at high static pressures, which reinforces the argument for high static calibration.

The twin post deadweight testers are widely regarded as an ideal industrial site instrument (as opposed to being purely a research and development or laboratory device), and have been used successfully, for many years, by Companies such as BP, Shell, Amoco, ConocoPhillips and British Gas in fiscal metering applications.

Additionally, differential deadweight testers are used as the primary standard in UKAS laboratories in the UK for certifying fiscal DP cells on behalf of the major oil and gas operators.

Example of a FISCAL flowmeter set-up

The flowmeter primarily consists of an orifice plate, or other take-off DP arrangement, fitted into the gas pipeline, with a DP cell fitted across the measuring point. The two static pressure cells, temperature cells, telemetry, and a flow computer perform the flow calculations.

Normally, the whole system is not allowed to exceed an overall 1 per cent inaccuracy. As an orifice plate can have an inaccuracy of around 0.8 per cent itself, it is extremely important that all the other elements of the fiscal set-up, (the pressure transmitter cells, the DP transmitter, the telemetry and flow computer), are as accurate as possible, to keep the whole system within the required 1 per cent.

Pressure calibration

It should be pointed out that the calibration of a DP cell can be carried out with one side open to atmosphere and simply applying a differential pressure to the other side. This is often used as a type of maintenance check to quickly see if the DP cell is functional. However, this method is not a true calibration of the cell and does not reveal the characteristics of the cell under its normal operating conditions.

Independent evaluation of the effect of line pressure on DP Transmitters in UK, the Netherlands and France over a five-year period has emphasised this significance, with a maximum zero shift observed of 6.9 per cent and a maximum span change observed of 4.2 per cent.

The main explanation given for the DP Transmitters output change, with line pressure, is the mechanical distortion of the wetted parts. Although this distortion of the cast body and the silicon filled measuring capsule is extremely small, the high degree of sensitivity means that this change is quite significant.

With care and the right approach to calibration, the uncertainty in differential pressure transmitters can be significantly reduced, potentially reaping huge savings.

[Brian Coan is the director & general manager — Australasia of DH-Budenberg Pty Ltd.]

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