Pump manufacturers and users are finding that hydrostatic leak testing of castings and pressure-containing parts before assembly is an excellent quality control measure. This ensures the integrity of products where leakage could cause premature failures, present a hazard to workers or the general public, or lead to fires or explosions.
These methods are not restricted to new pumps. While it is generally felt that pump testing is cost prohibitive for the small sized end user, it certainly can be used by shops that re-work, repair or remanufacture pumps of all types. Plants and refineries also fall into this category, as they may possess hundreds of pumps that require testing.
The downside for the small company is that they may employ many different types of pumps in their operations. This can make it difficult to justify an in-house test program for just a handful of pumps.
Why Leak Test?
When flow into and out of a pump affects its usefulness, leakage control is necessary. There are three basic reasons for leak testing: to prevent fluid loss that compromises system operation, to prevent contamination of the environment and to detect unreliable pump components and unacceptable leakage rates.
Besides the obvious safety benefits that come from implementing a long-term test program, product downtime for users can be significantly reduced. And there are these broader considerations:
- The U.S. government. Federal regulations are proliferating. Users are under increasing pressure to contain process fluids. With passage of the Clean Air Act, Clean Water Act, Resource Conservation and Recovery Act and other legislation, government officials are inserting themselves into the approval chain ahead of industrial criteria and customer needs.
- Professionalism. A testing system can combine functions of the assembly process, improve product line integrity and decrease production time. In addition, a pump testing program can be used as a sales tool—as long as it meets the exact specifications and objectives of the user.
- Troubleshooting. Pump testing can be an important part of on-site preventative and predictive maintenance programs in plants and refineries because computers allow on-the-spot statistical analyses from a database of pumps tested to pinpoint weak spots.
Aside from flow-rate and pressure tests conducted after assembly, leak testing tells more than just where the product leaks. It can also detect improperly tightened fasteners, incorrectly machined mating faces, missing gaskets or other elements, and other factors such as seepage due to casting porosity, holes, cracks, blisters or other defects in the casing.
Makers of pump and valve testing equipment have created machines that offer speed, sensitivity and dependability that the market now demands. The first specification when choosing a leak test system is what constitutes a satisfactory leak rate. This must not be confused with industry standards that call for hydrostatic testing of pump casings carried out for a length of time that allows complete examination of parts under pressure. Some test parameters are not considered acceptable unless zero leakage is observed for at least 30 minutes.
Leakage rates are best defined in terms of the amount of flow over a specific period of time at a constant temperature, i.e., pressure differential x volume/time.
For noncritical products, two rules are applicable: At 20 pounds per square inch (psi), a pneumatic leak rate of 12 standard cubic centimeters per minute (sccm) usually means a part will not leak hydraulic fluid or oil and at 20 psi, a pneumatic leak rate of 5 sccm usually means a part will not leak water.
Any leakage must be compatible with increased efforts to conform with current safety and environmental regulations for hazardous applications.
1. Bray, D.E. and McBride, D. Nondestructive Testing Techniques, 1992