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We have a problem with our on-line charge pump cavitating as we try to put an off-line pump in service after maintenance (common suction). We don’t seem to have a problem during normal swaps, only when we have to start it cold. We send resid feed to the pump for about 20 mins to warm it slightly before starting it then send it to a spill back line as we start it. We know that the pumps can handle any feed (gas oil, resid, etc.) as long as it isn’t a mixture, but for example when we switch from gas oil to resid during start-up we get the same cavitation problem. Anyone else having a similar problem or have any suggestions? It appears we have enough NPSH when starting the second pump, but we lose about 2.5 psi suction pressure and over 200 psi discharge pressure.
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I used to have that problem. I had a line installed at the suction side of the pump back to the Fractionator wash-zone so that I could warm-up the pump with the suction valve closed. After warming up the pump, you can then open the suction valve and close the line back to the wash-zone then wash this line with CLGO back to the wash-zone so that it doesn’t plug. This works every time. No more pump cavitations.
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Out of curiosity, did you ever figure out what the problem was? We are trying come up with an explination as well as a solution.
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Yes. Light material will back through the common suction line when you open the suction valve and cause the companion pump to cavitate. The trick is to warm up the pump prior to opening the suction valve. An added line back from the suction side of the pump to somewhere like the flash-zone can provide a means to warm up the pump.
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I agree with you, but in this case, we had the suction line open for over an hour when the pump cavitated. In our current procedure, when the pump is taken off line for maintanence, we begin by opening a line to blowdown and opening the suction line to warm to blowdown. This is when I would have expected to see a cavitation due to light material. The fractionator and blowdown DP is small, so we get limited warming this way. Next we open a spillback line to the fractionator and turn the previously off-line pump on and warm to the spillback line for a while before actually starting the pump. Right when we hit start on the off-line pump, the on-line pump cavitated. This would lead me to believe it is more of an NPSH problem, but during normal swaps, when the off-line pump is warm, we never have a problem. I know this pump is cold, but it is going to the fractionator and not to the suction of the on-line pump. Any thoughts?
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Limited warm up from the suction to the Blowdown could be a contributory factor but it seems as though from your biggest problem is while starting this pump there is a problem of some sort happening involving the discharge of the pump. Do you also reverse the flow through the discharge line through a warm-up line around the check valve as part of the pump warm up process prior to bringing the pump up on line? How hot do you get the case temperature before putting the pump on line?
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I agree that we only get a limited warm up by going to blowdown.
No, we do not go through the discharge valve through the warm-up line when putting the pump back in service after maintenance (for normal swaps we do this). We don’t do this because we were cavitating the pump by sending cold material through the suction line to the on-line pump. Instead, we open the min flow spillback to the fractionator feed line and flow from the suction of the pump we are trying to put on-line to this back to the fractionator. We do this so that we don’t send any cold material to the on-line pump. Obviously, something went wrong this time and we still cavitated the on-line pump.
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Interestingly enough, we have had similar problems with our charge pumps. We have a HCGO seal flush to the seals. As we return the pump to service after maintenance work, we fill the pump with HCGO to purge the pumpcase. We have experienced several “loss of flow” situations as we try to put this pump back on-line. We’ve attributed the cause to be reverse flow of the HCGO in the case, flowing back into the suction of the on-line pump, flashing and cavitating. We’ve been trying to proceduralize (is that a word?) the situation, but are considering some of the return/warm-up lines that you have mentioned. Coker Squirrel -Sounds like you’ve had success with the return line from the suction back to the frac wash zone. I’m assuming that you have a bypass around the discharge check valve and that you are cracking that discharge valve open to provide hot feed to warm the pump. Have you ever had this return line plug? Purging it with CLGO sounds like a good idea. Is your climate such that it requires some form of heat tracing?
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Yes, we have a 1 1/2″ line around the discharge check valve. And you are correct, we do partially open the discharge valve and crack open the 1 1/2″ valve around the discharge valve. We use the discharge from the companion pump through this line (1 1/2″ check valve bypass – we call it our “warmup line”) to slowly warm up the pump with the heater feed from the discharge of the companion heater feed pump. The key difference here is that we warm up the pump with the suction valve closed. We have installed a new line on the suction side of the pump (located close to the suction valve and within the pump isolation). This line goes up to the wash-zone portion of the Fractionator. If you completely warm up the pump with the suction valve closed through this line to the wash-zone, you will clear the pump and discharge line of all light material that may cavitate the companion pump through the revese flow on the suction line. After slowly warming up the pump this way up to around 400°F, you close the warmup line and then slowly open the suction valve. You then close the new line valve at the pump suction and then slowly open the warmup valve again. This puts the pump into a normal warmup standby mode and is ready to start.
I aslo have a wash header in which I use to clear the pump with LGO for maintenance, steam it out to the evacuation system and also to LGO wash all the associated lines after use. This is completely enclosed system, no hydrocarbon ever goes to the atmosphere.
Using this method, you can start your CHGO or CHLGO seal flush before warming up the pump with heater feed. This is recommended to keep the heater feed from ever entering the seals.
This works every time. No more pump cavitations. -
Marc, I see that I neglected to answer the other part of your question after posting. Yes, our climate is such that it requires some form of heat tracing. As part of your procedure, you should include to check the heat tracing to ensure it is in working order before taking the pump out of service for maintenance. The suction and discharge lines along with the warm-up line around the discharge valve should all be heat traced. Also, as part of your procedure and line modifications, you need an evacuation line off the top of your warm-up line. It is important to wash out the pump with LGO after isolating it to the evacuation system. If you wash through the warm-up line with LGO, you will not have any problems finding this line plugged when you need it to return the pump back to service. After, LGO washing the pump, you then follow it with a steamout in the same direction.
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