Section 5. PUMPS
1 PUMPS FOR HTW SYSTEMS.
1.1 Pump Requirements. Combination pumps are used in HTW combined pumping
systems to circulate water through both the HTW generators and the system.
The pump must provide the total head required to circulate water through the
c o m p l e t e circuit served. The head must overcome pressure drops through
v a l v e s , fittings, piping in the distribution system, and heat consumers. It
must also overcome pressure drops through HTW generators. Water pressure at
a n y point in the distribution system may be calculated as: expansion tank
pressure, plus static pump suction head, plus head developed by the pump,
m i n u s line pressure drop up to the point under consideration, minus difference
in elevation (static head) between the pump and the point considered.
4-3 illustrates a pump of the type used in HTW pumping systems.
1.1.1 Pump Head. The required pump head may range from 60 to 250 feet of
water above the HTW generator pressure, depending on the size and complexity
of the system. The pumps are normally designed with a fairly flat head
c h a r a c t e r i s t i c curve, to obtain a nearly constant head through the complete
load range. T h i s is important for parallel operation of pumps. The drop
between shutoff pressure and that corresponding to maximum operating flow
s h o u l d not exceed 15 percent of the shutoff pressure (zero flow condition).
1.1.2 Pump Type. P u m p s are of the centrifugal type, usually of single-stage
design. F o r systems with a high total head, a cost evaluation is usually made
t o determine the convenience of using additional single-stage booster pumps
a g a i n s t the use of multistage units. Pump casings may be of cast iron for
pressures below 150 psig; higher pressures may require cast steel
Pump internal components are usually made of stainless steel.
D e e p stuffing boxes should be used for sealing, with from five to nine packing
rings; or a mechanical type seal may be furnished. Adequate bearing and gland
cooling water systems must be provided, and drains should be visible.
1 . 1 . 3 Pump Location. P u m p s are usually located in the flow line of the
system, below and near the expansion tank. T h i s helps in maintaining the
highest possible net positive suction head (NPSH). The pump head should not
exceed 80 percent of the available NPSH in a plant arrangement.
1 . 2 Circulation Pumps. These pumps are used in HTW separate pumping systems
t o circulate water through the distribution system. Water is circulated
t h r o u g h the HTW generator by individual boiler recirculation pumps.
Circulation pumps are the same type as the combination pumps described above.
1 . 3 Boiler Recirculation Pumps. These pumps are used in HTW separate pumping
s y s t e m s to circulate high temperature water through the boilers. The design
i s similar to circulation pumps. T h e i r characteristics, however, depend on
t h e design of the boiler served.
1.4 Makeup Feed Pumps. These pumps deliver makeup water from the storage
tank or from a feedwater heater to the expansion tank. The pumps are usually
o f t h e e l e c t r i c a l l y d r i v e n , p l u n g e r t y p e ; the capacity depends on the time
r e q u i r e d to fill the expansion tank from the lowest to the highest permissible
o p e r a t i n g l e v e l . For a steady heating load, pumps may be started manually.