A flexible hydrostatic test system for the oil and gas industry must precisely measure a wide range of pressures. Because the items tested vary widely, most testing is done manually. A test engineer sets appropriate parameters, and a skilled operator adjusts the pumping system for the specimen under test. After the technician sets up the test, he or she is usually seated at the pressure source and controls the specified pressure. The human interaction lacks precision and repeatability; each time the test is performed, there is a variation in results. In extreme cases, the maximum rated pressure is exceeded, causing damage.
Automation separates the operator from the high pressure, narrows the variation from test to test, and eliminates damage from excessive pressures. With these goals in mind, Wilson Company’s Systems Division of Addison, TX has developed the IntelliTest® intelligent hydrostatic test system (Figure 1), a system that precisely applies the specified hydrostatic pressure and yields highly accurate pressure data across a very wide range.
A programmable motion controller is the key element of Wilson’s development strategy. This may sound strange since there is no actual motion required in the hydrostatic testing process; an electro-hydraulic motion controller excels at controlling pressure, and that control is the focus of the hydrostatic testing process.
Controlling and measuring pressure over a wide range is also a system requirement. A pressure transducer capable of measuring the entire desired range is not precise enough to measure small pressures. If a single transducer could cover the range, it would be prohibitively expensive. Instead, the Wilson team looked for a solution that incorporated multiple transducers, accurate over different ranges, and a means of seamlessly switching between them while controlling the hydrostatic pressure. The search brought them to the RMC75E motion controller (Figure 2) from Delta Computer Systems (Battle Ground, WA).
Hydrostatic testing involves using water to pressure-test a vessel. “Typically, we test up to 30,000 psi,” said Dave Pellerin, Wilson Company VP of Engineering, “but we can test to much higher pressures if needed.” In the Wilson system, air pressure controls reciprocating pumps that operate pistons to compress the water flow to the test specimen to the desired pressure: 100 psi of air going into each pump can produce 40,000 psi pressure out of the pump. The Delta RMC communicates with the pumps to turn them on and off via a built-in EtherNet/IP interface, and controls the fluid pressure being applied to the specimen via a proportional servo valve, interfaced directly to the controller’s ±10V analog output.
Pressure information is obtained from two pressure transducers, between which the RMC75 switches automatically, depending on the state of the pressure test being run. “For a large percentage of the time, we run open loop,” said Pellerin. “Then when we get close to the target pressure, the controller switches smoothly to closedloop control, running a user program embedded in the RMC75E to switch transducers based on whatever pressure criteria we decide to use.” The switching is managed by a unique feature called “custom feedback,” which can support switching between transducers of different capacity, or between identical transducers (to give the system a measure of fault tolerance should one transducer fail, or to enable averaging of sensor readings to increase sensor accuracy).
Feedback switching can be done on the fly by the controller. “We only need two transducers to handle the full range of 150 psi to 40,000 psi,” Pellerin said. “Most of our customers want the pressure being applied to be within a 5% tolerance of reading. Typically, we use a 40,000-psi pressure transducer during the operation, which is accurate to 4.8% at 1,500 psi, but if someone wants to test at 150 psi, the 40,000- psi version wouldn’t be accurate enough. For lower pressures, we use a 5,000-psi transducer.” Figure 3 shows the transducers connected to the motion controller.
The system uses multiple air-overwater pumps to provide high resolution of pressure across a greater range of pressures than could be produced with a single pump. The Wilson team programmed the RMC75 to switch from pump to pump as required. Given the controller’s ability to process digital I/O in addition to performing pressure control, the use of an additional PLC or industrial computer was not a requirement for this application (though some customers may add a PLC to download motion programs to the controller when necessary).