Page 349 - Boiler_Operators_Handbook,_Second_Edition

334 Boiler Operator’s Handbook

the motor load from the imbalanced forces. A feature of es as it slows down. Usually the motor starter will trip
the pump is control of the valves to vary capacity. The but there are many reports where the pump or piping
suction valve is held open on the discharge stroke (push- ruptured when someone accidentally started a pump
ing the liquid back into the suction) for varying degrees without opening all the valves in the system.
of rotation to vary the amount of water pumped. If you To prevent damage of that nature and motor start-
think it an antiquated way of doing things I can only say ers tripping or motors burning up a relief valve should
that the first nuclear merchant ship, the Savannah, had always be installed at the discharge of a positive dis-
one of those pumps for boiler feed. placement pump. If it’s reasonable to believe the flow
The only reciprocating piston pump you’ll nor- through the relief valve will always be of short duration
mally find in a modern boiler plant is a chemical feed then the relief valve can dump the liquid back into the
pump. Usually the piston is pumping a hydraulic fluid pump suction piping. It’s always possible that the pump
that transfers energy to the liquid being pumped using a could be operated for some time pumping the same liq-
diaphragm (Figure 10-70). uid and all the power will be diverted to heating up that
The capacity of a reciprocating pump is easy to liquid so it’s better, whenever possible, to route that liq-
determine. It’s equal to the area of the piston times the uid back to a tank or sump where there is a larger mass
length of stroke times the revolutions per minute if it’s of liquid to absorb the heat.
single acting. If it’s double acting, where the liquid is ad- A final note is appropriate before discussing spe-
mitted to and pushed out from both sides of the piston cific types of pumps. Any of them can be run backwards.
it’s twice that much less the cross sectional area of the Some, like centrifugals, can appear to operate, just not as
shaft times the length of stroke times rpm. well as with proper rotation. Gear and screw pumps will
Reciprocating pumps are positive displacement tend to pump the liquid in the opposite direction.
pumps. That’s a term we engineers use to mean that I’m reminded of the time I was asked to look at a
plastic, wood, metal, or whatever the pump is made of fuel oil pumping installation that, for whatever reason,
displaces (moves into the space that was occupied by) couldn’t produce more than 30 psig. After arriving at the
the liquid to move it through the pump. The steam pow- plant and introducing myself I looked at the oil system
ered duplex pump had some balancing features because with special attention to the pumps. There was one odd
the pressure on the liquid couldn’t exceed the difference provision, at least odd in my mind, because the check
between the steam supply and exhaust pressures times valves were on the suction side of the oil pumps. Check
the ratio of the areas of the pistons. That pump would valves are normally mounted on the discharge because
simply stop if the pressure on the liquid got too high. they will stop flow back to the pump if you stop it for
Motor driven pumps seldom simply stop, they produce something like packing failing. A pump with the check
very high pressures because the motor’s torque increas- on the suction side will not prevent leakage through
failed packing.
As I followed the lines to the boilers I noticed the
back pressure regulator in the overhead piping, found a
ladder to climb up and looked at the regulator and valv-
ing. I wanted to be certain the bypass valve had not been
left open but didn’t tell my escort that. When I got up to
where I could see I noticed the pressure gage at the inlet
of the regulator read 30 psig. After I made certain the
manual isolating valves were open, I opened and closed
the bypass valve. The gage still read 30 psig! When we
returned to the pumps I asked the escort to start one and
was informed that he couldn’t do it without an electri-
cian and there were no electricians on the job that day.
Dumfounded and concerned that I couldn’t learn
much more without operating the pumps I was asking
him what happened when they tried to run the pumps.
He claimed they made a lot of noise and some oil leaked
out of the check valve as he pointed at the seam at the
Figure 10-70. Piston chemical feed pump bonnet of the valve. When I told him that the pump was