Pump Warm Up Procedures And Hot Alignment 3

[longeron]

After a quick search of the forum I did not see much discussion of start-up procedures for hot pumps.

My question is three-fold regarding warm up:

How quickly do you allow pumps to be warmed up? I've heard that most OEMs suggest about 100F per hour to allow even thermal expansion.

How do you warm pumps? I've heard about warm-up lines that bypass discharge check valves, holes drilled in check valves, and operators cracking the suction with a line to process sewer off the discharge. How do you recommend regulating the speed at which the pump is warmed up?

How far along in the warm-up procedure do you suggest performing a "hot alignment"?

 

[JJPellin]

The heat-up rate should be specified by the manufacturer. But, every one I ever looked up was 100 °F per hour maximum. The criticality of this depends on the specific pump. We have some fully lined slurry pumps with hard chrome liners that tend to crack if heated up too fast. We have some vertical sulfur pit pumps that will snap the shaft off because of differential growth between the shaft and column if you heat them up to quickly.

We use at least four methods for heating up hot service pumps. Each has it own issues and depending on the configuration of the pump, one may be superior to the others. For a typical single stage overhung pump, we prefer to have a piped warm-up line from the discharge that connects into the bottom of the case. This will tend to give the best, most uniform heat up. If the pump is top suction, top discharge and you drill the check valve or bypass the check valve the flow can short-cut from the discharge line back up the suction line and may not heat the case full to the bottom.

With some of our big barrel pumps, it is defiantly better to have a bypass piped around the check valve. We can’t drill a hole in the check valve because we would either get too much flow or we would have to drill a very small hole that would plug off. With a piped bypass, we can include a multi-orifice stack that can take the higher pressure drop without eroding or plugging off. Otherwise, we could install a globe valve in the bypass to regulate the heat up rate. We do not want to put the warm up line to the bottom of the case on a barrel pump. The case drain is in contact with the pump suction on some of these pumps. So, once again, the flow could short-cut and not heat the entire pump up. By bypassing the check valve, the flow has to pass through all stages of the pump to get to the suction line.

For vertical in-line pumps, vertical turbine pumps or two-stage, between bearings configurations, a drilled check valve often works very well. This is also our second alternative for single stage overhung pumps. As long as the drilled hole is at least 3/16”, and the service is clean enough to not plug that up, it works well.

We have standards that require that the pump temperature is within a certain differential from feed drum temperature before the pump can be started. This varies from pump to pump. But typically it is 40 °F to 80 °F.

I can’t answer your question about hot alignment since our policies do not allow it. It is very difficult to do a good hot alignment with energy control policies that require a block valve blocked and chained and all other sources of energy blocked and locked. By the time we establish full energy control and get a work permit, the pump will no longer be hot. Instead, we do cold alignments with pre-determined offsets to allow for thermal growth. If you are still allowed to do a hot alignment with open valves, then the pump should be within about 50 °F of feed drum temperature when you align it.


Johnny Pellin

 

[electricpete]

We hit the same problem with hot alignment as J Pellin.

The alternative is to use some type of equipment like Permalign to measure change in alignment from off-line to running, then factor that info into future cold alignment targets.

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[longeron]

JJ, Electricpete, one end-user I worked with had a list of pumps and the expected thermal growth offset for each asset. Their millwrights were expected to take that offset into consideration during alignment. That seemed to work pretty well- of course if the offsets are poorly calculated the results would also be poor.

The other consideration with thermal growth is internal clearances. API 610 suggests an additional 0.005" on top of the minimum diametral clearances. One end-user I worked with allows 150% of the allowable clearances that 610 suggests in hot pumps. They also do not tend to follow the OEM warm-up recommendations. I suppose it may be possible that there is some convenience in that; however, that convenience must be paid for in efficiency and shaft support.

How does a hole in a check valve play with the process safety folks?

Operators actually sit next to the pump with an IR temp gun monitoring the temperature rise and adjust the warm-up line?

 

[JJPellin]

We have no process safety issues with a drilled check valve. It is a back-flow path, but the flow is so low, that we have not seen any situations where it could cause a significant process safety risk. Most of our warm up lines are orificed so they don't have to make ongoing adjustments. In some rare instances, they warm up the equipment by flowing through to slop with a hose coming off a drain or vent connection. In those, yes, they compare temperatures using an IR gun and watch the time to stay within the heat-up rate limits called out in the procedure.

I should comment that the drilled check option is almost always used with pairs of pumps installed in parallel. One pump is running and the other is being warmed up (or kept warm). In this instance, there is no back-flow other than the short loop through the off-line pump. In other words, any flow backwards through the pump being warmed up passes up the suction line, crosses over into the running pump and is pumped forward into the process. It does not backflow all the way to the suction vessel. If the pump is unspared, there would typically not be adequate pressure in the discharge system to back-flow for warm up. We could not use a drilled check for an unspared charge pump in a hydrotreater, for example. We could not allow high pressure hydrogen to back-flow into the feed surge drum which is not rated for hydrogen service. A pump in this service would have to be warmed up using a line to slop passing hot feed forward through the pump.

We are able to validate our calculated thermal offsets values at each repair. If the vibration data shows that we have a good running alignment, then the targets must be good. If the vibration shows a residual misalignment problem, then we have to change the offset values.


Johnny Pellin

 

[GPRnD]

What type of pumps are you looking at ?

The allowable rates for thermal warm up vary greatly on the type of pump. An OH2 or a BB2 can handle much higher ramp rates than a typical BB3.

 

[david6245]

Hello,

This is an interesting topic for me as I am in the process of sizing a RO in a 1" warm up line around the discharge check valve of a pair of hot (415 deg F) condensate pumps. I have a somewhat simple question: is backflow through these pumps in this type of application an issue? The flow I calculated for this warm up line is 7 gpm vs a pump capacity of 1500 gpm. Are pumps designed to handle back flow up to a certain limit?

Any info would be great.

Thank you,

David

 

[vpl]

David please ask this as a separate question.

Patricia Lougheed

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