Page 358 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 343

rotors plus the end of the idler rotor bearing against the get the liquid up to pressure, but a necessary means of
casing. Some manufacturers use an enlarged end on the controlling the flow.
rotors to increase bearing surface. Other schemes include When we’re dealing with on-off pump control
balancing lines between suction and discharge ends ap- there are opportunities to improve that method of con-
plied to balance pressure forces. Another scheme is oppo- trol to reduce energy costs and wear and tear on the
site hand ends of the rotors so they draw liquid from both pump. An attitude of limiting the number of starts by
ends and discharge in the middle to balance the hydraulic extending run time is one you should adopt. I’m not
pressures almost completely. talking about recirculating liquid to keep the pump run-
Screw and gear pumps do not do a very good job ning, that saves starts but also wastes energy. It is an
of pumping compressible fluids. An oil pump can easily option you should consider if the pump has extremely
get air bound where there is a sufficient volume of air or short off cycles where it may run for ten minutes then
vapors at the discharge and inlet to expand and contract shut down for five to ten seconds. If that’s the case then
as each cavity between rotors is opened and closed, thus recirculating some liquid to keep it running past those
preventing any flow through the pump. The air also short off cycles will save on pump and motor wear and
leaks back just like the oil. It’s not uncommon for the reduce wear and tear on the starter as well.
pump to generate a loud audible roar when air or vapor Every time the pump is started the entire assembly
is trapped in it because the air or vapor doesn’t do a very is subjected to stresses above and beyond the normal
good job of lubricating the pump and it forms bubbles in operating conditions. Motor current is five to ten times
the oil as it leaks back. normal operating current when the pump is started and
Operating a screw or gear pump with a vapor trap those high currents produce rapid heating of the motor
for any extended period of time will ensure complete windings with attendant thermal stresses and also high
breakdown of the film of lubricating oil on the rotating magnetic forces that can dislodge the windings. The run
parts with subsequent damage to the pump as the metal then stop and run then stop operation is also rough on
parts start rubbing. It’s necessary to vent them to elimi- bearings, both in the motor and in the pump, because
nate compressing air or vapors in the pump that will the bearings will heat up and cool down with some
prevent liquid entering. Properly vented the pump will breathing that can increase the probability of air mix-
move air to eliminate it from the suction piping. ing into the grease or oil to corrode them (see Chapter
When starting a dry pump (filled with air) it’s 6, Lubrication). The pump always experiences pressure
important to ensure that lubricant film is maintained. spikes when starting because the liquid in the connect-
Making certain some of the piping is full of oil that will ing piping has to be accelerated from its stationary
be drawn into the pump is important to limit wear. The position and the check valve has to be lifted. By reduc-
oil is also a sealing film that helps the pump trap the air ing starts you’ll reduce the strain on the equipment to
in its cavities and push it through. The best way to do it extend its life.
is to fill the suction strainer with oil, shutting down at Reduce starts by stretching the pump’s on-off set-
regular intervals and repeating the process until all the tings as far as you can. Let the level get a little lower and
air or vapor is pumped out. Once you have a suction line run the pump until it’s a little higher by adjusting the
full of liquid the pump will work. level controller. There are limits to this, including allow-
ing the level in a boiler to get so low that a little upset
Pump Control results in operation of the low water cutoff. All that re-
There was a time when the only control we had ally does is tell you where the low limit is.
over the operation of a pump was to turn it on or turn I doubt if you can take advantage of that to get
it off. That’s still a common means of controlling the something replaced. I ran into one operator that was
pumping of liquids, used almost exclusively for feed- starting and stopping a pump repeatedly keeping his
ing low pressure boilers and returning condensate but hands on the enclosure with his two thumbs on the start
modern technology has expanded our abilities and the and stop buttons. When asked about it he responded
wise operator should know how to utilize those meth- “I’m trying to blow this damn thing up so they’ll give me
ods. Note that this involves controlling the pump to one that works.” On a repeat visit to the plant I noticed
control the flow. Before variable speed pump control we a new identical pump had replaced it. The operator was
typically controlled the fluid flow to maintain operating still grumbling because he got the same make and model
parameters. Many times that meant recirculating the pump back so it still didn’t do what he wanted it to do.
liquid that was pumped, wasting the energy we used to As for controlling the flow through a pump auto-