CE I:lEFRESHER
•. .
...
PERFORMANCE of a typical series of centrifugal pumps
and detailed characteristics
of
the selected pump-Fig. 1
flowrates, the throttling valve has to open and pass larger flows with less resistance. In manufacturers' manufacturers' head-capac head-capacity ity curves curves,, the "Total "Total Head" usually does not include velocity-head differences calculated between suction and discharge flanges. Those diagrams that include velocity-head differences indicate "Total Dynamic Head" on the vertical scale. In this case, velocity-head velocity-head differences differences should be added add ed to the total
The curves in this drawing suggest that when the dis charge is throttled, the system-resistance system-resistance curve moves to so! Only one system-resi another position. position. Not so! system-resistance stance curve curve exis exists ts for a given given discharge discharge line.' The. varying.M' of the control valve valve brings the system system resistance resistance up to a point on the head-capacity curve. Head-capacity Head-capacity curves curves are. drawn for. for. pumping pumping water. water. Howeve However, r, a centrifuga centrifugall pump with given impeller, speed and size will will develop develop the same head in feet, no matter matter what what the the liquid liquid or spec specif ific ic gravity-providing viscosit viscosities ies are normal. Tile Tile static-he static-head ad pressure at the pump nozzle nozzless will will be.higher be. higher with a h ~ a v i e r liquid, an the horsepow horsepower er requirement requir ement will will also increa increase. se. Brak Brake e horsepower requirements for p u 1 t l p i n g w a t e r a r ~ super" imposed imposed on the head-capac head-capacity ity curye(seeFig.lb). We multiply the horsepower reqpired t o p ~ m p ~ a t e r b y .the specific specific gravity gravi ty of the liquid being pumped to obtain actual ..• brake. horsep?we r,. Effic Efficie ienc nc Cllrves are •..... also superimposed on ,the head-capt;lcityCUt'Yes(Fig.l b). The emideal, ideal, normal. o p e r a t i t l g p o i 1 1 t i s i t t . } h ~ m ~ u m emciency
UNIT ,pressure losses In new, commercial-steel pipe help in selecting suitable sizes for discharge piping-Fig. 4
Pressure the drum, 1'1' is lOpsi,and pressure pressure in the column, ,i 15 psi. Pressure drop across the control valve, AP equals psi; and the friction losses in the I psi and Ap'i = suction and discharge lines are Apl 9 psi, respectively. respectively. Let us calculat calculatee the required total head for the pump, i.e., i.e., the differential differential pressure, pressure, !:J.P, between the suction and discharge flanges. Both suction head and discharge head consist of four componen components: ts: static static head, pressure pressure head, veloci velocity ty head, head, and piping piping and. components components resistance. Veloci Velocityty-hea head d differences differences normally do nOt have to be calculated because they are taken into account by U.S. pump manufacturers manufacturers in their published. published. head-capacity head-capacity curves. curves. Pressure Pressure and static static hea d in Ii systemusuaIly remain constant constant with changing capacities.. capacities.. SuctionSuction-pipe pipe,, discharg discharge-pip e-pipee and
components resistances resistances vary with flowrate flowrate (see (see Fig. Fig. 2). The analysis, analysis, therefore, becomes: becomes: Suction, Psi
==
CHEMICAL ENGINEERINGlMAY
26,
1975
Vessel pressure Static-head pressure Friction loss
Total
PI
PHI
=
/),PI
=
Discharge, Psi
10
P 2
5
Pm
.::i 14
/),P2
= 15 = 10 =-.2. 34
now subtract subtract the total total suction suction resistance resistance from the We now discharge discharge line backpressure totind the· pump's differ differ ential pressure excluding excluding the control valve: valve: 34 - 14 20 psi. Choosing a c o n t r o l ~ v a l v e !:J.Pcv of 5 psi, we must add this to the pump's differential pressure of20 psi to obtain the total differential pressure across the pump flanges as
=
117
