Pump hydrostatic pressure testing (hydrotest)
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By E. M. Araza
CENTRIFUGAL PUMPS – Modern Design and Practices
Pump hydrostatic pressure testing is a process in which the major pressure-containment components of a pump are pressurized to a predetermined level and held at that pressure for a specified period of time in a carefully controlled and safe environment.
Hydrostatic testing is performed to:
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The terms hydrotest and hydrotesting are short for hydrostatic testing. These terms are used interchangeably.
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The major pump components that normally require hydrostatic test are the casing, cover, bowl, nozzle head, stuffing box, and outer barrel or can. These parts should be tested as assemblies whenever applicable. The test may also be required for other components, such as cooling jackets or cooling coils, auxiliary piping, etc., but typically at a lower pressure. (As used in this article, the term casing is generalized for simplicity and may include those components.)
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Hydrostatic testing is normally done at a pressure of 1.5 times the pump MAWP unless a safety factor (SF) other than 50% is agreed upon with the end user or is allowed under an applicable standard. Example, in some pipeline installations a 25% SF may be allowed if the pipeline has the same SF.
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Segmental hydrostatic testing refers to the practice of testing pump components under different test pressures whereby an area normally exposed to suction pressure only is tested at 1.5 times the maximum suction pressure, whereas the area normally exposed to MAOP is tested at 1.5 times MAOP. Examples are the pump nozzle head, and barrel or can. This is to avoid over-designing the component that will result to excessive metal thickness, heavier component weight, and higher equipment cost.
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An indication that a pump have been segmentally hydrotested is when its flanges have different pressure ratings – for example, it has a 150 lbs suction flange and a 300 lbs discharge flange. The pump flange rating does not necessarily indicate the actual hydrostatic test pressure/s - this information should be taken from the test report.
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Segmental hydrostatic testing has fallen out of favor because in an upset condition, such as in a sudden power loss or reverse pump rotation, or in the event of a sealing element failure, the entire pump can be exposed to the MAOP. Another disadvantage is that it can result in significantly different metal thickness and thermal stress/expansion between the low and high pressure areas of the pump.
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MWP vs. MAWP
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The term maximum working pressure (MWP) is also known as maximum operating pressure (MOP). These terms are used interchangeably.
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The maximum working pressure (MWP) is not to be confused with maximum allowable working pressure (MAWP). MWP refers to the expected maximum working pressure the pump is going to operate at, whereas the MAWP refers to its design pressure limitation. MWP is system-related whereas MAWP is pump design-related. The system MWP may be equal to or less than the pump MAWP, but in no case shall MWP be higher than the pump MAWP.
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Some general guidelines
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As a rule-of-thumb, the recommended hold time for the hydrotest pressure is one hour for every ?” of wall thickness - with minimum of one hour and maximum of four hours.
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For safety reason, the pressurization should be done incrementally and held at those incremental pressures for a short period of time before full pressurization is achieved. The incremental pressurization can be done at 1/3, 2/3, and full pressure rating .
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Components to be hydrotested shall have their critical fits pre-machined only prior to the test. Final fits machining shall be done only after the components passed hydrotest as there is a possibility that some critical fits may become out of specs after the hydrotest.
According to API 610 “by allowing a small amount of material to remain at these critical areas during hydrostatic testing, the necessity of adding material by welding to restore close-tolerance dimensions after hydrotest is avoided.”
In most hydrostatic testing tap water is used as the test medium. In some instances where the pumps are designed to handle very low specific gravity liquid, kerosene or another liquid may be used as test medium to better simulate the pumps’ operating environment. Wetting agent or dye may be added for better visual examination.
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The test must be conducted in a secure, safe, and restricted area to be accessible only to? authorized test personnel who must wear individual protective gear. Failure to adhere to this safety measure may result in serious or fatal injuries in the event of a test failure, as has been reported many times in the past.
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If the component passed the hydrotest a tag or stamp shall be made and attached to the unit to show that it passed the test. A test certificate shall be issued, if required. If the component failed the test an NCR shall be issued, and the necessary corrective actions should be made based on approved procedures or methodology.
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Determining maximum working pressure (MWP)
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The maximum working pressure (MWP) of a pump is normally calculated based on its maximum shut-off pressure, at maximum specific gravity, and maximum running speed.
The maximum shut-off pressure is the maximum differential pressure of the pump at zero flow for stable curve, or the maximum pressure at the hump in a pump with a drooping curve, plus the maximum suction pressure.
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Some users specify that the maximum shut-off pressure shall be based on the maximum impeller diameter for the pump, not on its rated diameter. Also, some industry standards specify a 5% head allowance be added for pumps driven by constant speed drivers or a head allowance based on 5% overspeed for pumps with variable speed.
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Determining hydrostatic test pressure
The required hydrostatic test pressure is most commonly at 1.5 times the pump’s MWP, for 50% service factor (SF). This is true when the pump is handling a liquid at relatively low temperature. Example, a pump of cast steel material that will handle a liquid at 150 degrees Fahrenheit with an MWP of 400 PSIG has to be hydrotested at 600 PSIG. But if the pump were to handle a liquid at elevated temperature, a material temperature correction factor must be applied to compensate for the lower allowable tensile stress of the material at that elevated temperature. This is important specially when using materials, such as stainless steel, whose tensile strength degrade quickly with increase in temperature. At elevated temperature, the required hydrostatic test pressure (P) should be calculated as follows: ??
P = (MWP / F ) x 1.5 ?
where F is the material temperature correction factor and is equal to the ratio of the allowable stresses of the material at the higher operating and at hydrotest temperatures. (These values are taken from ASME stress table.) Example: What should be the hydrostatic test pressure of the above mentioned pump if it were made of cast stainless steel, SA-351, CF8M316, and if it were to handle a liquid at 650 degrees Fahrenheit? Solution From ASME stress table, its allowable tensile stress is 17,500 PSI at ambient temperature (up to 100 degrees) and 11,500?PSI at 650 degrees Fahrenheit.
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P = (400) / [11,500/17,500]) x 1.5) = 913 PSIG
It is usually advantageous for the user to buy the pump based on the vendor’s standard hydrotest pressure, rather than to specify a lower but non-standard value. In this example, if the vendor’s standard hydrotest is 1200 PSIG, there is no economic advantage to be gained in specifying a lower 913 PSIG hydrotest pressure for that pump. On the other hand, a hydrotest pressure of 1200 PSIG will increase the pump MAWP to 526 PSIG – 126 PSIG higher than its MWP of 400 PSIG – thereby increasing its SF.
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Hydrostatic test report
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There are many forms of Hydrostatic Test Reports from pump companies depending on their specific needs for information. The following is a consolidation of the information shown in those reports:
Customer’s name
Name of test conducting facility
Pump type, size, and serial number
Pump part name, part number, or other traceable identifier
Part description (optional)
Part material or Heat number
Applicable test procedure (customer or vendor’s procedure, HI, API, etc.)
Test date, start time, stop time, actual hold time
Design or rated pressure
Actual test hold pressure
Pressure gage number or identifier (optional)
Pressure gage range (optional)
Pressure gage calibration due date (optional)
Case or component volume
Test temperature
Test liquid or agent - water, kerosene, wetting agent
Pass or fail checkbox (if test failed, describe failure mode or issue NCR)
Name of test operator or technician
Name of test witness, if required (optional)
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The test report should include the casing volume (the author is a strong proponent for its inclusion as a line item on the pump data sheet.) This is for future use to determine the volume of liquid to be removed in draining or decontaminating the casing when the pump is taken out of service, or the volume of liquid needed for any re-hydrotesting. It is also used to estimate the allowable time the pump can run with closed discharge control valve without exceeding its allowable temperature rise.
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It is common practice for the test reports to include a certifying statement such as:
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“This is to certify that the above item is hydrostatic pressure tested in accordance with (name of company or applicable industry standard) procedure and meet, or exceed, all hydrostatic test specifications required per the customer’s purchase order.”
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Additionally, according to API 610, “any areas that are machined after hydrostatic testing shall be identified on the hydrotest report.”
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?When re-hydrotesting is required
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Re-hydrotesting of a pump, or its pressure containment component, should be performed if any of the following conditions existed:
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·?????? After they underwent repair or modification that affected their pressure boundary.
·?????? If their operating pressure and/or temperature increased.
·?????? If their material of construction changed.
·?????? If the metal thickness has been reduced due to corrosion, erosion, or cavitation.
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Hydrostatic Testing of Old Casing
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Re-hydrotesting of an old pump casing needs special attention to ensure that its structural integrity has not been compromised due to aging. It is possible the casing thickness could have been reduced due to corrosion, erosion, fitting, or cavitation. Or worse, it might have reached its casing retiring thickness. Metal fatigue or loss of elasticity might also compromise the structural integrity of the casing. This could be checked by providing dial indicators at critical areas of the casing to probe for any deformation or movement during the hydrotest. The occurrence of such deformation or movement is an indication that it is time to replace the old casing.
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About the author
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The author has more than 40 years of work experience in the global pump industry, with four different companies, in various capacities including as pump department manager, senior design engineer, and hydraulic specialist. During his career, he was involved in the design, manufacture, assembly, testing, troubleshooting, upgrade, and hydraulic re-rate of thousands of centrifugal pumps, both in the new equipment (OEM) and aftermarket sectors for which he received many engineering achievement awards.
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He conducted pump trainings, seminars, and webinars internationally. He wrote several technical articles on centrifugal pumps and had developed? ?a software on pump hydraulics and mechanical analysis. He received his pump studies and trainings in the U.S., Canada, Belgium, and Japan. He has a BSME degree, was a licensed professional mechanical engineer, and was a former member of ASME. He is now retired and created the LinkedIn group CENTRIFUGAL PUMPS – Modern Design and Practices. To join the group, or to access more technical articles on centrifugal pumps, please visit https://www.dhirubhai.net/groups/14199776/
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You can contact the author by sending an email to [email protected].
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Rotating Equipment Engineer at Medco E&P Natuna
3 个月Nice article. Thanks for sharing
head of rotating equipment department in oil design and construction company(ODCC)
4 个月good article but the additional points are: 1- maximum discharge pressure can be increased to trip speed of driver . 2- minimum MAWP of API pumps except BB1 and vertical types are 40BAR at 38 C 3- the chloride content of liquid test for SS shall not exceed 100ppm.
NDT, bolting, heat treat, pumps , motors, etc Consultant
4 个月Go Sergio