Many pump startups are the culmination of months, if not years, of work to design the process, machine or system; specify components, instrumentation and protective devices; and review and qualify suppliers. It is also the most vulnerable time for any pump. Part I of this article describes cautions, reviews and inspections for three main components that should be conducted before startup to help ensure that gremlins of pumping systems are identified and eliminated. In a future issue, Part II will give additional tips on how to prevent delays and other, last-minute pointers to ensure successful startup of rotary pumps.
Piping & Valves
Be sure that all the required valves have been installed. Verify that none are installed backwards. An absent or reverse-mounted check valve, foot valve or relief valve can cause serious damage. Piping should be cleaned and inspected after fabrication to ensure that weld beads, weld rod, scale, wombats and any discarded tools/materials have been removed. Small, hard weld beads can cause catastrophic pump failure should they become lodged in the wrong pump clearance.
Ideally, the entire piping and valve system will be thoroughly flushed to remove all dirt and fabrication debris. This is customarily conducted with a flush pump, not the normal system pump. Strainers and/or filters are installed at appropriate locations, and dirt accumulation is monitored until they show no accumulation for a period of 24 hours. Flushing usually uses light, fairly hot (150 F/65 C) oil delivered at flow rates higher than the system design. The higher flow rates cause higher liquid velocities within the piping system and are more likely to dislodge debris.
Some systems use vibration equipment to impose mechanical “shaking” on the piping, again to maximize the dislodging of dirt. Extensive piping systems have been known to show debris accumulation even after 30 days of flushing (hence the need for the startup strainers discussed later). Because of their long distances and relatively huge holding volumes, pipeline systems frequently use bullet-shaped pigs, sometimes equipped with wire bristles, which are propelled ahead of a flush or initial product batch of liquid to scrub debris and dirt from the inside of the pipe.
If permanent pump inlet strainers are not part of the system, a temporary strainer must be installed. It is not unusual for supposedly clean suction piping to dump damaging debris into an expensive pump. Pump manufacturers can easily locate the source of debris from the subsequent damage. Rest assured such failures are seldom (if ever) covered under warranty.
The strainer sizing and mesh used are critical to it successfully performing in a startup. The recommended commissioning (temporary) strainer mesh size depends on the fluid viscosity and is:
- Viscosity Use 60 mesh (0.010 inch, 0.25 millimeter [mm] opening)
- Viscosity >450 SSU (86 cSt): Use 40 mesh (0.017 inch, 0.42 mm opening)
- Viscosity >1000 SSU (216 cSt): Use 20 mesh (0.033 inch, 0.84 mm opening)
Operators should continuously monitor the pressure drop across the suction strainer and limit the pressure drop across the strainer to 10 pounds per square inch (psi) or 0.7 bar, or the vapor pressure of the pumped fluid.
Consider the system flushing to be complete when the strainer pressure drop stabilizes, and periodic inspection of the strainer confirms that any accumulation of debris has stopped. At this point the commissioning (temporary) suction strainer can be removed.
The piping system should be pressure tested and should never exceed the design limits of any system component. For example, some pumps can withstand discharge pressure only on the discharge side and inlet piping may be a lower pressure class. Verifying the allowable maximum pressure on the general arrangement drawing or equipment nameplate is a good practice to help ensure the pressure test about to be undertaken will be safe and not damage equipment.
The pressure test medium should be compatible with the components/system to be tested. For example, do not use water if the system is not a water system. A low pressure (15 pounds per square inch gauge [psig]) compressed air test may be adequate to find missing flange gaskets or other obvious leak sources.
Pump inlet and discharge piping should have been made up from the pump for a distance of perhaps 20 feet (6 meters) to minimize pipe strain on the pump. Piping should also be independently supported.
Contrary to the opinion of many piping engineers, the pump is not designed to be a pipe anchor. Never draw piping into place at the flanged connections of the pump. This can impose dangerous strains on the unit and misalignment between the pump and driver. Pipe strain adversely affects the operation of the pump, which can result in damage to the equipment and is potentially a safety risk. Limit the allowable piping misalignment to those in Table 1.
Check and tighten all flange bolts to specified torque.
If the criteria in Table 1 cannot be achieved, rework the piping until they are. Do not just run all the nuts and bolts tight with an impact wrench and hope for the best. Hope is not an acceptable strategy for successful, reliable system operation.