Reassess your Pump Startup (S/U) and Shutdown (S/D) Procedures By Heinz P. Bloch For TECÉM tecem.com.br Oct,05,2013 It may sound far-fetched, but the ramifications of incorrect pump S/U and S/D procedures have recently been debated in reliability-related litigation in U.S. courts. At issue was the performance of multi-stage centrifugal pumps in pipeline service at a location that had to limit motor inrush currents. The pumps operated above their first rotor critical speed and operation at the critical speeds had to be limited to two or three seconds in duration. Should the discharge block valves be fully or partially open, or should they be closed? No one rule fits all As can be seen from our four-part illustration (Figure 1), the power demand of impellers with different specific speeds can increase or decrease as the flow increases. Radial flow impellers (part A, with specific speeds up to about 120 metric or 6,200 U.S.) have power curves (P) that increase with flow. This would favor starting against a fully closed discharge valve. The same would be true for the mixed flow pumps of parts B and C, with their specific speeds from 40 to about 200 (2,000-10,000 U.S.). Obviously, their power slopes are also increasing with flow. It is different with the axialflow pumps of part D. Their specific speeds typically range from 160 to about 350, or 8,000-18,000 U.S. These axial flow pumps are best started with the discharge valve fully open because they exhibit lower power demands at lower flows. It should be realized, however, that pumpage pushing against a fully closed discharge valve will often make it near impossible to open the valve. The recommended procedure would thus call for starting against a partially opened discharge valve. Partial opening in this context means a valve gate that has just been lifted off its seat. The pumped medium thus wets both upstream and downstream sides of the sliding gate. Review and update your procedures Reliability-focused pump users consider using the following procedures: A. Starting Centrifugal Pumps 1. Arrange for an electrician and a machinist/specialist to be present when a pump is initially commissioned. Ascertain that large motors have been checked out.
2. Close the discharge valve and open the suction valve. Except for axial flow pumps, the almost closed discharge valve creates a minimum load on the driver when the pump is started. Assuming that the motor inrush current allows and that the motor will not kick off, the discharge valve may be just "cracked" --about 1/8 open -- before the pump is started. 3. Be sure the pump is primed. Opening all valves between the product source and the pump suction should get product to the suction, but does not always ensure that the pump is primed. Only after ascertaining that fluid emissions are not hazardous or are routed to a safe area, open the bleeder valve from the pump casing until all vapor is exhausted and a steady stream of product flows from the bleeder. It may be necessary to open the bleeder again when the pump is started, or even to shut down and again bleed off vapor if pump discharge pressure is erratic. Note: Priming of a cold service pump may have to be preceded by "chilldown". A cold service pump is one that handles a liquid that vaporizes at ambient temperatures when under operating pressures. Chilling down of a pump is similar to priming in that a casing bleeder or vent valve is opened with the suction line open. There are three additional factors to be considered for cold service pumps: Chilling a pump requires time for the pump case to reach the temperature of the suction fluid. The chill-down vents are always tied into a closed system. On pumps with vents on the pump case and on the discharge line, open the vent on the discharge line first for chill-down and then open the pump case vent to ensure that the pump is primed. Should it be necessary to have a cold service pump chilled-down and ready for a quick start, e.g., refrigerant transfer pumps during unit startup, then the chill-down line can be left cracked open to get circulation of the suction fluid. 4. If a minimum flow bypass line is provided, open the bypass. Be sure the minimum flow bypass is also open on spare pump if it starts automatically. 5. Never operate a centrifugal pump without liquid in it. 6. Check lube oil and seal pot level (assumes dual seals). 7. Start the pump. Confirm that the pump is operating by observing the discharge pressure gauge. If the discharge pressure does not build up, stop the pump immediately and determine the cause.
8. Open the discharge valve slowly, watching the pressure gauge. The discharge pressure will probably drop somewhat, level off and remain steady. If it does not drop at all, there is probably a valve closed somewhere in the discharge line. In that case, close the discharge valve. Do not continue operation for any length of time with discharge valve or line blocked. 9. If the discharge pressure drops to zero or fluctuates widely, the pump is not primed. Close the discharge valve and again open the bleeder from the casing to exhaust vapor. If the pump does not pick up at once, as shown by a steady stream of product from the bleeder and steady discharge pressure, shut down the pump and driver and check for closed valves in the suction line. A dry pump will rapidly destroy itself. 10. Carefully check the pump for abnormal noise, vibration (using vibration meter) or other unusual operating conditions. An electrician and machinery engineer should be present when pumps are started up for the very first time, i.e. upon being initially commissioned. 11. Be careful not to allow the bearings to overheat. Recheck all lube oil levels. 12. Observe whether or not the pump seal or stuffing box is leaking. 13. Check the pump nozzle connections and piping for leaks. 14. When steady pumping has been established, close the startup bypass and chill-down line (if provided) and check that block valves in minimum flow bypass line are open. B. Watching Pump Operation 1. Especially during commencement of pumping, but also on periodic checks, note any abnormal noises and vibration. If excessive, shut down. 2. Note any unusual drop or rise in discharge pressure. Some discharge pressure drops may be considered normal. When a line contains heavy, cold product, and the tank being pumped contains a lighter or warmer stock, the discharge pressure will drop when the line has been displaced. Also, discharge pressure will drop slowly and steadily to a certain point as the tank level is lowered. Any other changes in discharge pressure while pumping should be investigated. If not explainable under good operating conditions, shut down and investigate thoroughly. Do not start up again until the trouble has been found and remedied. 3. If lubricated by oil mist, periodically check oil mist bottom drain sight glass for water and drain, if necessary.
4. On open oil mist systems, regularly check both oil level and oil mist vapor flow from vents or labyrinths. 5. Seal oil pots need to be checked regularly for correct level. Refill with fresh sealing liquid (oil, propylene glycol or methanol, as specifically required and approved for the particular application). 6. Periodically check for excessive packing leaks, mechanical seal leaks, or other abnormal losses. Also, check for overheating of packing or bearings. Do not trust your hand, use a surface pyrometer instead. Note that excessive heat will cause rapid failure of equipment and may result in costly and hazardous fires. 7. At a minimum, operate all spare pumps at least once a week to prevent the bearings from seizing and to ensure that the pump will be operable when needed. Realize, however, that Best Practices Plants have determined monthly switching of the A and B pumps to be the most appropriate and costeffective long-term operating mode. 8. When a pump has been repaired, place it in service as soon as possible to check its operation. Arrange for a machinist to be present when the pump is started up. C. Shutting Down Centrifugal Pumps 1. Close the discharge valve. This takes the load off the motor and also may prevent reverse flow through the pump. 2. Shut down the driver. 3. If pump is to be removed for mechanical work, close the suction valve and open vent lines to flare or drain as provided. Otherwise leave suction valve open to keep pump at correct operating temperature. 4. Shut off steam tracing, if any. Continue oil mist lubrication, if provided. 5. Shut off cooling water, sealing oil, etc., if pump is to be removed for mechanical work. 6. At times, an emergency shutdown may be necessary. If you cannot reach the regular starter station (in case of fire, for example), stop the pump from
the starter box, which is located some distance away, and is usually accessible. If neither the starter station nor the starter box can be reached, call the electricians. Do not, as a part of regular operations, stop pumps from the starter box. Use the regular starter station instead. Please note that these procedures are of a general nature and may have to be modified for non-routine services. Review pertinent process data, as applicable. Re-write in concise sentences if instructions are to become part of checklists which operators are asked to have on their person while on duty. Figure 1: The power demand of impellers with different specific speeds can increase or decrease as the flow increases. High-flow low-dead pumps ( D ) should be started with the discharge valve fully open. This article was written and compiled by Heinz P. Bloch, a reliability improvement consultant with over 52 years of industrial experience in equipment reliability and failure avoidance. He is the author or co-author of 18 textbooks and over 550 conference papers and articles on related subjects. His latest books on pumps and compressors were published in 2011 and 2012. All are available from Amazon.com.