Proper pumping critical for successful hydrostatic testing.
As the term suggests, the portion of the pipeline under test is filled with water and pressurized beyond the maximum operating pressure (MOP) designated for that line and then held at that pressure for a pre-determined time period. Under the governmental mandates, test pressure of 125% of the MOP is required continuously for four hours and then further testing at 110% of MOP for another four hours, or as specified in the testing specifications for the pipeline.
If, during the test, the line breaks or leaks, an immediate pressure drop occurs. In that instance, the segment where the leak occurred is identified, excavated, repaired and retested.
Pumps Get Workout
Pipeline testing normally uses one or more centrifugal pumps for filling. It is a unique application for this equipment as these pumps are normally designed for applications with defined discharge and suction conditions. In the testing process, however, the pump is required to go through its complete range of conditions.
From the filling of an empty pipe to the line pack-off, the pump runs through its entire operating capacity of zero head, maximum flow to maximum head, minimum flow. Even though these conditions are largely uncontrollable, there are certain principles that, if followed, can result in better service and lower costs.
One of the most important considerations is to match the pump system with its intended use. Each component, including suction hose and foot valve, low head pump strainer, flow meter and fill pumps, should be matched to the flow requirements. To properly size and select a pump system for application, the following four factors must be determined:
* Capacity--This would be rated in U.S. gallons per minute (GPM) or U.S. gallons per hours. Conversions to imperial gallons (Canada) = U.S. gallons x 0.8327 and from liters = U.S. gallons x 3.7854.
* Pressure = Expressed in pounds per square inch (psi) or feet of head. With atmospheric pressure at sea level equivalent to 14.7 psi or 33.9 feet of water column. One psi is equal to 2.31 feet of water column. Its metric equivalent is BAR. One BAR is equal to 14.5 psi. In calculating the total pressure, it is the summation of suction lift, discharge elevation, friction loss and discharge pressure. The pressure is commonly referred to as feet of head, whether it is negative on the suction side or positive on the discharge.
* Supply Source--Water source can be from a well, lake, river, ditch or tank. Each source has a bearing on the suction conditions and the type of pumps required.
* Location Of Supply--The pump's location can have a bearing on whether it will have a positive or a negative head. The distance of the line being tested from the pump can create a loss of head due to friction. An understanding of the four factors present and knowledge of the final requirement of flow will determine the pump selection. In pipeline testing, it is not always possible to explicitly determine all of the required data. The experience of the test foreman and his crew can play a large part in proper pump selection and in conducting a successful test.
Typical Hydrostatic Test Spread
In order to cover all contingencies, a qualified and knowledgeable source of hydro test pumps, related equipment and supplies should be available. This source should have a range of standard pump sizes available that can be combined into various arrangements to meet the broad spectrum of testing requirements. This combination of pumps, arranged with ancillary equipment in a spread designed for the specific locale and operating conditions, will provide the most efficiency and cost-effectiveness for the testing process.
Typically, the hydrostatic test spread used for major pipelines will be comprised of these various components (Figure l):
* Low Head Pump--This unit will typically be used to draw water from the most viable source, whether it is a river, tank or suitable reservoir.
* Strainer--The water pumped through the low-head pump has to be strained to remove solids and foreign particles. A 100-mesh screen is normally used.
* Chemical Injection Once strained, the water is further treated with a chemical injection system that can include pumps for adding one or more chemicals, such as rust inhibitors and oxygen scavengers, to the test water.
* Flow Meter--The filtered and treated water is then pumped through a flow meter. A record of the flow rate, in GPM, is used to determine the amount of flush water or the volume of water behind the test pig in order to calculate speed, location and the time necessary to fill a section of the pipeline under test.
* Fill Pump--From the flow meter, the water goes into the pipeline through the fill pump. Depending upon the static head that the pump will be facing, the fill pump can vary in capacity and arrangement, which can differ with each section tested. Therefore, the configuration and combinations of fill pumps are not standard, but vary with application. Where needed, multiple pumps can be mani-folded to produce increased volume or pressure. Multiple pipeline sections can be filled from a single source. If a section is being dewatered and transferred to another section, a reservoir should be placed in the outlet of the dewatered section to keep air from being entrained into the next section and also to eliminate the shock to the pumps being used.
* Pressure Pump Once the line has been completely filled with the fill pumps, the hydrostatic pressure testing can commence using the pressure pump. The pump is fed from its own reservoir or water supply and can be set up in a completely different location from the other pumps in the spread. The amount of water is dependent upon the various sizes of pipe, how well the line has been filled, the volume of water needed to overcome compressibility and the amount of pressure required for the test.
[FIGURE 1 OMITTED]
A high-pressure hose is run from the pressure pump to a flow meter and from the flow meter to the check valve installed at the entrance valve to the pipeline. The pressure pump is furnished with a transmission or variable speed drive to regulate the volume of water being pumped while the pipeline is being smoothly brought up to test pressure. The flow meter is used as a cross-check to make sure the valves are seating properly and is producing the flow per stroke when plotting the rise in pressure per stroke.
At this point a high-pressure flexible hose is run to a test manifold where a series of valves and instruments are used to accommodate the test instruments (Figure 2). The use of this remote manifold reduces the possibility of worker injury if there should ever be a line rupture.
[FIGURE 2 OMITTED]
Higher Elevations Present Challenges
One of the most demanding testing situations occurs in mountainous terrain. In addition to high head conditions, water sources are often not readily available. This sets up a scenario in which it is critical to conserve water and pump it long distances at high pressures.
Higher elevation pumping often necessitates staging of a number of pumps to reach the final height. In this situation, low head pumps in the chain can be subject to over pressuring. It is recommended that additional relief valves and check valves, along with those already installed on the pumps, be placed in the line.
Due to the type of water being pumped at higher elevations, a faulty check valve or debris under the valve supplied on the fill pump can allow a back flow from the static head on the line. In that situation the back flow may be greater than the lower head pumps are designed to handle. Because of this potential problem, it is advisable to have additional check valves in the line in the event that the pump is shut down or if one pump in the series fails. This check valve redundancy allows for the removal of a pump, if necessary, or for line failure, while conserving the water that has already been pumped (Figure 3).
[FIGURE 3 OMITTED]
Hydrostatic testing has proven to be a useful method of validating the serviceability of a newly constructed pipeline or the integrity of an existing line. However, the usefulness of this testing method depends on the quality of the pumping and testing equipment, the design of a spread suitable for the operating conditions and the knowledge of the test crew. When these three factors work in harmony, the hydrostatic test will have its highest success rate.
Author: Jim Hodde is president of Power Associates International, Inc. in Houston. He has a B.S. degree in engineering from the University of Houston and is a Registered Professional Engineer in the state of Texas. He has more than 30 years of experience in the design and leasing of pumps and associated equipment for use in hydrostatic testing. Since 1978 his company has supplied a variety of equipment for pipeline projects worldwide.
James Hodde, P.E., Power Associates International, Houston
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|Comment:||Proper pumping critical for successful hydrostatic testing.|
|Publication:||Pipeline & Gas Journal|
|Date:||Jan 1, 2007|
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