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Failures and trouble shooting of Boiler feed Pump
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  Boiler Feed Pump Issue thread407-336105 Share This Forum   Search   FAQs   Links   JobsWhitepapersMVPs   Read More Threads Like This One hunter321 (Mechanical) 21 Dec 12 6:48 Hi, i am having an issue with one of our boiler feed pumps. Randomly the pump is tripping due to low outlet flow (this is a protection in out control system). The pump is a Torishima multistage (10 stage) horizontal axis and is fed from a deaerator. I've chcked all the temperatures/pressures and the water is not at saturation. We've installed some instrumentation and the first thing noted before the pump tripping is a drop in balance line pressure, this is followed a few seconds later by a drop in suction pressure and then outlet pressure. All vibrations are fine. The suction strainer has been checked and is clean. If anyone has experienced this problem before or has some suggestion i would be very interested to hear. Thanks you, Mark.  pennpiper  (Mechanical) 21 Dec 12 7:03 Just thinking: a) are there two 100% pumps or three 50% pumps?  b) does the number and location of the pumps cause more fittings in the suction line of the subject pump? c) are the suction lines to all of these pumps undersized?  prognosis: Lead or Lag hunter321 (Mechanical) 21 Dec 12 7:08 Hi pennpiper. a) Two 100%. Standby pump starts and keeps process running when duty pump trisp.  b) Not sure what you are asking here c) probably should have said. THe pumps were installed five years ago and have ran without issue until a few weeks back. So pipes are sized correctly. JJPellin (Mechanical) 21 Dec 12 7:51 If these were my pumps, I would gather data and evaluate the pump performance. I would verify that they were still performing well relative to the srcinal test curve. I would ask our instrument techs to check the proper operation of the level control on the deaerator. An intermittent loss of level could explain the problem.  You said that the first indication was a drop in balance line pressure. I assume you mean  balance line differential pressure. Does the pump use a balance disk or a drum? Does the pump have a thrust bearing or does it depend on the balance device? If it has a thrust bearing, is the  bearing spring loaded like some of the big Flowserve barrel pumps? A shift in axial position could change the clearance in a balance disk arrangement and change the balance line differential pressure. Is your vibration data from a continuous monitoring system? Are you actually looking at vibe data during the event? Or, are you just looking at vibe data taken by a portable system during normal operation? It sounds to me like you could be losing level in the deaerator because of a faulty level controller. At some point, you draw vapor into the pump suction. The vapor passing through the pump causes a loss of performance and affects the operation of the balance disk or drum. The lag in indication could be related to the sample rate of the DCS data points. We could provide better input if you could provide a flow schematic and describe your control system in detail. A more complete description of the pump would be helpful to me since I am not familiar with this manufacturer. Johnny Pellin hunter321 (Mechanical) 21 Dec 12 8:51 Hi JJPellin, I'll try answer all your questions. - We've checked the pump performance and it looks fine. - The deaerator level is also fine. This is verified by three level transmitters. DCS is 1 second interval, level is flat before, duraing and after the event. - The balancing devide is a disk. - We also have a thrust bearing, i don't think is is spring loaded, balancing device takes most of the axial forces. - The balance line pressure is not differential. The balance line comes off the final stages of the  pump and feeds back to the deserator. There is a pressure control valve on the line and a standard pressure gauge. - Our vibrations are continuously monitored, as i said, they all look fine prior to the event. - Some pump details below Pump manufactures TORISHIMA Product Number AP345059  Number of supply 2 Type &Size Single suction mul ti-stage diffuser horizontal  No of Stages 10 Rotation CW  Design Pressure 218.7 barG Pumping Liquid BOILER FEED WATER Total Head HP 1670 m Total Head IP 605 m Capacity (Discharge) 320 m3/h Capacity (Extraction) 110 m3/h Speed 2980 Pump Efficiency. 79%  NSPH 8.8m Pump ing Temp. 140.4 DegC JJPellin (Mechanical) 21 Dec 12 10:18 Thank you for the additional information. I don’t understand what you are describing as a balance line. I am familiar with a balance line as the line that takes the leakage from the balance disk or drum and routes it back to the pump suction line or supply vessel. What you described is a line coming off the discharge of the  pump and spilling back to the DA. I would describe this as a minimum flow spill-back line. A  balance line would not normally have a pressure control valve in it. A spill-back line would have some sort of flow or pressure control. So, just to clarify, is this a balance line (based on my definition) or a minimum flow spill-back line? Rather than wait for an answer, I will address both options. If this is a balance line , then I can’t imagine why it has a pressure control valve in the line. I don’t understand what parameter would be used to tell this valve to open or close. But, in any case, I would expect that there is some problem with the control of that valve. If some signal (false indication or true) tells that valve to open, then the “balance line pressure” would drop. With lower pressure on the downstream side of the balance disk, the pump might thrust in reverse (depending on the thrust bearing arrangement). The faces of the balance disk would open up and the leakage through the balance line would increase. With more flow through the  balance line, the flow available for the outgoing process line would be reduced. Then low flow is registered and the pump trips off. If this is a minimum flow line, the scenario is very much the same. The pressure control valve on the spill-back line seems like a strange choice. I would normally set the minimum flow control based on flow, not pressure. But, if this is trying to control the outgoing stream to a constant pressure, I guess this could work. If this pressure control were to malfunction, the  pressure control valve on the spill-back line could open, diverting flow back to the DA. The outgoing flow to the process line would drop and the pump would trip on low flow. This assumes that the pump would trip on low outgoing flow and not total pump flow (spill-back flow plus outgoing flow). I am not familiar with any sort of an extraction pump with a single suction and two discharges. I assume that is what you are describing. So, there could be something particular to this type of   pump that I know nothing about that could explain your problem. If this pump has all of the impellers in one orientation, then the pressure on the balance line is very, very important for  balancing the high thrust. If the impellers are in an opposed configuration, then the net thrust would be much lower and the balance line pressure would not be as critical. Johnny Pellin hunter321 (Mechanical) 21 Dec 12 10:31 Sorry, maybe i misled you, you are correct the balance line is taken from the balance disk and routed to the DA. The pressure control valve on this line is a manual valve. Its is set during commissioning and should not need to be changed. It is usually set about 1-2 bar higher than the DA pressure. It basically sets the pressure at the balance disk. In our case it is fully open, so in effect is doing nothing. Our balance pressure is a bit on the high side, about 3 bar higher than the DA, the OEM does not think this is a problem. I can't understand how the balance line  pressure can drop suddenly, unless the balance device sticking or passing excessively. Thanks for your help. JJPellin (Mechanical) 21 Dec 12 10:59 The picture is getting clearer. I have never seen a pressure control valve in a balance line, but we don’t have any boiler feed water pumps for pressures this high. This would be most comparable to our big coker jet water pump (270 m3/hr, 248 barg, 2570 kW, 10 stages) A drop in balance line pressure (given a fixed restriction in the balance line) suggests a reduction in balance line flow. That suggests that the faces of the balance disk closed which would normally happen for thrust movement in the active direction. A drop in suction pressure would not be easily related to a change in thrust position. This is especially true if the thrust occurs first and is followed by the drop in suction pressure. You mentione d a suction strainer. I don’t like to have suction strainers at my boiler feed pumps. Unless the DA is chronically sending scrap iron to the pump, it should not be needed. Unless there was a clearly understood problem that required a suction strainer, I would remove it. The drop in discharge pressure comes after the drop in suction pressure. This makes even less sense to me. I can see a situation where a drop in discharge pressure causes an increase in thrust load which closes up the balance disk which drops the balance line flow which reduces the balance line pressure. But, I can’t come up with an explanation that works the other way around. Do you monitor thrust position? Does the thrust position shift during this series of events? Otherwise, I am stumped. Sorry. Johnny Pellin hunter321 (Mechanical) 21 Dec 12 11:38

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