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
