Pump Warm Up Procedures And Hot Alignment

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"?  
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. 
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.

From the original1Check Valve.com