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PuK - Process Technology & Components 2022

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Pumps and Systems Leak

Pumps and Systems Leak testing on progressing cavity pumps New process developed for safe and reliable leak testing on progressing cavity pumps using compressed air Method decreases the risk of accidents and reduces the use of drinking water by saving 10 l per pump and test Dipl.-Ing. (FH) Johann Vetter The unique rotor-stator geometry of a progressing cavity pump makes it impossible to fully dry out the pump after a conventional leak test with water. The water-based test process can partially wash away the corrosion protection on the rotor. A subsequent complete application of preserving products due to the thread effect is hardly possible. This can lead to corrosion on standard pumps, which are often made of carbon steel. Another disadvantage water-based testing is the moisture that often remains on the floor of the test benches, where it poses an increased risk of accidents for the staff. This prompted the manufacturer NETZSCH Pumpen & Systeme GmbH to develop a compressed air solution for leak testing. The new process and the change of the test medium eliminate the slipping hazard for employees and the risk of corrosion. At the same time, the use of corrosion protection products can also be omitted. In addition to this, 10 l of drinking water are saved per pump and the test period was reduced from 1.5 to 5 min. The German company has been using the new test solution for pumps of the sizes NM 003 to NM 063 since August 2021 and the process is already used at the factory in Goa/India as well. When it comes to leak testing on progressing cavity pumps, the standard procedure is to introduce water under pressure over a defined period and then check for leaks. The manufacturer from Waldkraiburg in Germany, for example, tests its pumps at 7 bar for 5 to 15 min as a 100-% test, i. e. a leak can be 100 per cent excluded upon successful completion of the test. But this method also has drawbacks, so the pump manufacturer consequently set out to develop an alternative, settling on compressed air testing in March 2021. had to be made gas-tight, the relative differential pressure had to be determined and the appropriate filling and holding time had to be defined for each pump type or size. To find the correct specification and produce a practical test solution, Fig. 1: When it comes to leak testing on progressing cavity pumps, the standard procedure is to introduce water under pressure over a defined period and then check for leaks. Solution for compressed air testing developed in a pilot system The company based its development on other sectors such as the airbag industry. In the airbag manufacturing process, airbags are leak tested with gas as a standard. Two of the benefits of this procedure are that no media is transferred and that the test can be completed much more quickly. The latter is because the gas has a lower density than air, allowing leaks to be detected faster and with greater accuracy. However, the different viscosities of the two media also presented the manufacturer with challenges during the development of a test method. For example, the pumps to be tested the company experimented with a pilot system for the series-production assembly of small pump. Around 200 comparison tests between water and compressed air were conducted until the experts settled on 500 mbar test pressure, a maximum pressure drop of 50 mbar and a test duration of 25 s for the compressed air test based on this empirical investigation. In the second step, the successfully tested solution was transferred to a production test bench for the medium series with sizes up to NM 063 and verified successfully. Depending on the pump size, the test is conducted with different test programs to reach the corresponding overpressure. In addition, the 38 PROCESS TECHNOLOGY & COMPONENTS 2022

Pumps and Systems Leak testing on progressing cavity pumps pressure in the complete interior is measured with a pressure gauge on the opposite side of the test. This ensures that the pressure is present during the whole test. For the test, the pump is always equipped with a flange on each intake side and delivery side, while the leak test device is connected to the intake side with a compressed air coupling and the pressure gauge is connected to the delivery side. The parameters for the pressure test have to be set according to the pump type and size. For an NM 063 progressing cavity pump, for example, we fill the pump for 80 s, let the medium settle for 10 s, test for 25 s and then it takes 30 s to release the pressure. The subsequent test is automated. If the result is OK, the pressure gauge and the leak test device can be disconnected and the flanges can be removed. If the test results indicate a leak, however, a sniffer is used to determine the pressure difference in the pump, followed by a repair and re-testing of the pump. Fig. 2: After successfully completing of the test, the water has to be drained from the pump on the test bench. It runs onto the workshop floor and then through drainage channels into the sewer system, creating wet surfaces that are a slipping hazard and accident risk for personnel. Comparison with conventional testing with water One advantage of testing with compressed air over conventional testing with water is that the corrosion protection inside the pump remains fully intact after the test. Before the stator is installed in the progressing cavity pump, the rotor is covered in oil or grease, and this preservative can be partially removed and flushed away by the water during the test. After the test, the water can be expelled from the pump with compressed air and a preserving agent is applied to protect the unit against corrosion. But one problem remains: The test medium cannot be fully removed from inside the pump during drying out, which is due to the operating principle of progressing cavity pumps. This is similar to a screw thread, i. e. any liquid in the thread turns cannot be fully removed due to the cavities and the adhesion. A similar principle applies to applying preserving products: A relatively high level of corrosion protection can be achieved in the intake and delivery sides, as the remaining water is easy to remove there. However, inside the pump, the thread effect means that it is also harder to

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