Standard of a Pressure Measurement
The selection of a pressure calibration standard is a critical decision in the success of many industries. If the standard is not sufficiently accurate, significant losses both of revenue and/or process control can occur. On the other hand, if the standard is excessively accurate for the task being performed, cost of the standard may be too high and the labor time to calibrate may be high. It is important, therefore, for the potential user of a pressure standard to investigate both the needs of the task requiring the pressure standard and the overall costs associated with each pressure standard that is being considered.
7.1 Application Considerations
Many different types of pressure calibration standards are available. In order for the user to select the correct tester, several aspects of the task to be performed should be considered.
7.1.1 Test Fluid
An important element is the test fluid. Since the test fluid will enter the pressure sensing element of the instrument being tested, the test fluid must be compatible with the process fluid to which the instrument will be attached. Otherwise, all instruments must be cleaned after testing, an expensive operation. The most common test fluid is instrument grade mineral oil. Where useable, oil provides an outstanding combination of corrosion resistance with lubrication of the close fitting parts of the pressure standard. Distilled water also provides an excellent test fluid which is inert to most process fluids. Clean, dry air or nitrogen gas however, eliminates the problem altogether.
7.1.2 Pressure Range
A survey should be made of the pressure range of all instruments to be tested. The pressure standard should be capable of producing pressures in excess of the highest instrument to be tested. Care must be taken to consider the accuracy of the pressure standard at the lowest pressure to be tested. Several pressure standards of different ranges may be required to maintain acceptable levels of accuracy.
7.1.3 Task To Be Performed
Consideration should be given to the task to be performed. If most of the instruments to be tested are fixed in place, such as recorders, transmitters, etc. the portability of the instrument is important. If air transportation is required, such as oil/ gas platforms, the size and weight is important. If many instruments are to be tested, dual column dead weight testers which change test range quickly should be considered. If many technicians will use the tester, the tester should be rugged and relatively independent of operator technique. High performance tasks, such as testing of instruments at manufacture, require custom designed testers.
7.2 Cost of Measurement
The most important consideration in the selection of a pressure standard should be the potential loss of revenue resulting from the use of the instrument. If the pressure standard is used to measure the quantity of product either being purchased or sold then the accuracy of pressure measurement can be directly related to the profit or loss to the user. In other instances, the pressure measurement is related to the efficiency of a piece of equipment or process and can again be directly related to profit or loss. Another use of pressure standards may be the test or calibration of safety related equipment. In this instance the consequences of failure should be considered when selecting the pressure standard. On the other hand, some pressure measurements are made for routine maintenance activities with minimal consequences of failure. Each of these considerations will be discussed in more detail in the following paragraphs.
A Worksheet for determining the economic analysis is shown on Figure 7-1. This worksheet can be used to evaluate various instruments for any desired application.
7.2.1 Custody Transfer
The custody transfer involves the use of a pressure standard to determine the quantity of a commodity to be purchased or sold by a customer. A typical example is a natural gas pipeline wherein all natural gas entering the pipeline is being purchased and all gas leaving the pipeline is being sold. The quantity of gas in each transaction is determined by a measurement of flow across a meter, usually an orifice plate.
For purposes of this analysis the flow rate at a single metering station is assumed as 1,000,000 cubic feet per day at a 25″ H2O test point, the price of natural gas is estimated at $2.00 per 1000 cubic feet.
The equation for flow through an orifice meter is:
Flow = C x SQRT(ΔP x Ps)
C = Flow constant
P = Differential pressure over the orifice plate
Ps = Static pressure at the meter
At the 4″ H2O calibration point, the flow rate is:
Flow (4″H2O) = 1,000,000 Ft3/Day x SQRT (4″ H2O/25″ H2O) = 400,000 Ft3/Day
As computed in Chapter 6, the overall accuracy of the deadweight tester at 4″ H2O was 0.069% and at 25″ H2O was 0.069%. The overall accuracy of digital (A) at 4″ H2O was 43.1% and at 25″ H2O was 6.986%. For digital (B) the accuracy at 4″ H2O was 1.1345% and at 25″ H2O was 0.24032%.
The estimated measurement accuracy losses per test station for these three examples are therefore:
A. Deadweight Tester
Error (4″ H2O) = 4″ H2O x 0.069% = 0.00276″ H2O
Error (25″H2O) = 25″H2O x 0.069% = 0.01725″ H2O
Loss (4″ H2O) = 400,000 – 400,000 SQRT (4 – 0.00276)/ SQRT(4) = 138 Ft3/Day x 365 days x $2.00/MCF = $100.74 per Year
Loss (25″H2O) = 1,000,000 – 1,000,000 SQRT (25 – 0.01725)/SQRT (25) = 345 Ft3/Day x 365 Days x $2.00/MCF = $251.85 per Year
B. Digital Electronic Tester (A)
Error (4″ H2O) = 4″ H2O x 43.1% = 1.724″ H2O
Error (25″H2O) = 25″ H2O x 6.986% = 1.7465″ H2O
Loss (4″ H2O) = 400,000-400,000 SQRT (4 – 1.724)/SQRT (4) = 98,272 Ft3/Day x 365 Days x $2.00/MCF = $71,738.56 per Year
Loss (25″ H2O)= 1,000,000 x 1,000,000SQRT (25 -1.7465)/SQRT (25) = 35,562 Ft3/Day x 365 Days x $2.00/MCF = $25,960.99 per Year
C. Digital Electronic Tester (B)
Error (4″H2O) = 4″H2O x 1.1345% = 0.04538″ H2O
Error (25″H2O) = 25″H2O x 0.24032% = 0.06008″ H2O
Loss (4″ H2O) = 400000-400000 SQRT (4-0.04538)/SQRT (4) = 2275 Ft3/Day x 365 Days x $2.00/MCF = $1660.75 per Year
Loss (25″ H2O) = 1000000-1000000 SQRT (25-0.06008)/SQRT (25) = 1202 Ft3/Day x 365 Days x $2.00/MCF = $877.46 per Year
7.2.2 Process Control
Process control involves the use of a pressure standard to measure pressures that are critical to the efficiency and/or control of processes. An example of this type of application is the measurement of feed water pressure in steam powered electric generating plants. The operating efficiency of the plant is affected by the accuracy of measurement of this pressure. Other applications involve the uniform control of pressures within a process to assure optimum operating efficiency.
The worksheet for determining the economic analysis of various pressure standards is used in the identical manner as that illustrated in Section 7.2.1 except that the algorithm for process efficiency is substituted for the orifice plate flow algorithm.
Pressure standards are frequently used to test and/or calibrate safety related control systems.
These systems may be designed to prevent overpressure within tanks or lines, overfilling of tanks, etc. with the potential of severe environmental damage, employee injury, etc.. If the pressure standard is not sufficiently accurate, the safety system may be certified as accurate and malfunction in the event of an emergency.
Using the three pressure standard examples from Section 7.2.1, consider a safety application wherein the actuation pressure is 4 inches of water column with a set point tolerance of one percent. The maximum pressure tolerance acceptable would be 1% x 4 inch Water Column or 0.04 Inches of Water. If a 4:1 margin of safety is required, only instruments with .04/4 or 0.01 inches of water accuracy would be acceptable.
The deadweight tester (0.069%) would then be the only acceptable instrument for the test.
Many pressure measurements are made only to verify that the operating pressure recording devices are operating. Accuracy of measurement is generally not a major concern for this type of application. Of concern may be the size, weight, portability and cost of the pressure standard.
As an example, consider that a customer has an application that utilizes process gauges with accuracy of +3.0%, and a minimum range of 25 Inches of water. The actual error of this equipment would be 3.0% x 25 inch Water Column = 0.75 inch Water Column. Of the three examples evaluated in Section 7.2.1 both the deadweight tester (0.069%) and the digital gauge (B) (0.24032%) would be acceptable solutions. The final consideration would then be the size, weight, portability, ease of use and cost of each instrument.
7.3 Summary and Conclusions
Many different factors must be considered when selecting a pressure standard. The customer must consider the requirements of the task to be performed, the overall accuracy of the pressure standard for the application, any specific application considerations including the test fluid, pressure range and the task to be performed, the cost of inaccurate pressure measurement either on the purchase or sale of product, the adequacy of the pressure standard to test or adjust safety related systems or specific requirements for routine maintenance inspections.