Pumps and Paralell/Series Flow
Pumps and Paralell/Series Flow
(OP)
I regularly get involved in pumping several circuits (Both heating and cooling) utilising centrifugal pumps. I am looking for a good technical book or information on assessing the interaction of circuits and the effects of residual pressures. I studied this at Uni many years ago but my notes are knowhere to be found.
Worked examples would be a great help.
Typical scenarios are:
a. three or more pumps in paralell pulling water through a boiler or number of boilers.
b. Three or four chillers in paralell each fitted with integral pump sets.
etc.
Drapes
Worked examples would be a great help.
Typical scenarios are:
a. three or more pumps in paralell pulling water through a boiler or number of boilers.
b. Three or four chillers in paralell each fitted with integral pump sets.
etc.
Drapes





RE: Pumps and Paralell/Series Flow
Given a pump curve head (horizontal) vs flow (horizontal).
For pumps in series, the curves add vertically. (Add the heads for a given flow).
For pumps in parallel, the curves add horizontally (add the flows for a given head).
(The above assumes no significant cavitation effects.)
When you have the new pump curve, find intersection with the system curve to determine the operating point.
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RE: Pumps and Paralell/Series Flow
You could download Epanet software and set up some models. Preferably you could invest in AFT's Fathom and model them far easier. This will enable you you to check out the NPSH, pump curve vs system curve, do what if scenarios. If you also get their add on modules then you can goal seek, carryout extende period simulations. Go to www.aft.com and check it out.
RE: Pumps and Paralell/Series Flow
Following are a few tips on multipump/multibranch fluid system design based on my experience
1. Use centrifugal pumps of the same design (same rated head and flow) in all branches to equalize pressure rises in all operating branches and to enable abnormal pump characteristics to be determined from single-pump tests.
2. Select pump designs to have a specific speed that will minimize adverse effects of abnormal operations, particularly under reverse flow conditions. Pump specific speeds in the range of 2000-4000 (US units)are preferred.
3. If calculated idle branch reverse flows will breakaway an idled pump and/or cause reverse rotor rotation at speeds higher than the forward rotation maximum speed, employ quick closing check valves in the branch lines that may be idled. Swing check valves should have disks that are fully backseated at normal system flowrates to avoid flow fluctuations in the system due to oscillating disks. At lower flowrates (with idled branches or reduced pump speeds) determine check valve disk angular positions and evaluate disk flutter tendencies and fluid system repetitive check valve slamming prospects (easier said than done). Use seat bypasses around closed check valve disks if temperature near-equalizations between idle and active branches is a concern with respect to pressurized thermal shock damage to piping or system components. Seat bypasses should be sized to reduce pressure surges in idled branches without causing excessive backlow that might compromise system heating/cooling requirements.
4. Selected pump design point flow should be based on the normal number of running pumps to meet 100% system rated heating/cooling capacity. That is, operation of more or less pumps are non-normal operating conditions and account needs to be taken of degraded pump performance particularly for below rated pump flows where high impeller inlet flow incidence angles can cause flow separation, stationary or rotating stall ands other undesirable flow conditions that might induce system flow instabilities.
5. Use pressurized flow systems with operating pressures sufficiently high at all pump running speeds and flow fractions to avoid two-phase (liquid-fluid vapor) flows that can damage system components by cavitation or cause system 2-phase flow instabilities. Establish and rigorously maintain minimum pump suction pressure requirements for all pump speeds and operating branch conditions (ie. 1,2,3,4, etc. branches operating).