FACTORY TRAINED OTJ EXPERIENCE ENGINEERING SUPPORT
Characteristics of a Good Pumping System
Guide to Minimizing Start-up Headaches
There is nothing quite like that anticipation prior to starting-up a newly
installed pump system, hoping that everything will go as planned. The
following article highlights installation recommendations and pump accessories
that will contribute to a reliable pump system.
This is not an in-depth catch-all article covering all types of pumps,
rather the following information summarizes recommendations by Wanner
Engineering, manufacturer of sealless positive displacement pumps for general
industrial and precision metering applications. As such, these guidelines
are especially relevant for all positive displacement pump designs. What are Positive Displacement Pumps? They are pumps that discharge a fixed volume of fluid at a given pump
shaft RPM regardless of the inlet or outlet pressure. Centrifugal pumps
use centrifugal force to add to the inlet pressure and having a flow rate that
changes (decreases) in relation to increased discharge pressure. Inlet Condition Surmising there is adequate flow to the pump is easier when pumps are fed
by a pressurized source, such as a water main or pressurized tank. Pump Reservoirs Pumps fed by a supply tank need sufficient volume to enable entrained air
to escape. This is particularly important when the supply tank is
frequently filled or fluid is bypassed back to it, such as the bypassed flow
from a pressure-regulating valve. It is always a good idea to route such
feed lines into the tank as far away from the tank outlet as possible and to
introduce the fluid below the low level area to prevent extra air entrainment.
Using oversized inlet plumbing reduces the inlet velocity and resulting
agitation that contributes to entrained air and in some cases foaming.
Entrained Air Creates Problems Positive displacement pumps displace a fixed volume of fluid per pump
shaft revolution, ASSUMING a non-aerated fluid is being pumped. Entrained
air or other gasses make the fluid somewhat compressible and that coincides with
variation in fluid displacement. Protecting the Pump System
If you require filtration to protect your pump or
equipment downstream, it is a good idea to filter on the inlet
side of the supply tank. When strainers or filters clog, they
reduce the flow at a given pressure. Restrictions on the inlet
side of a pump system contribute to cavitation; a potentially
violent imploding of entrained air bubbles resulting in powerful
shock waves harmful to both centrifugal and positive
Inlet cavitation is a common pump problem and often associated with
clogged filtration or plumbing components restricting flow rate to the pump.
Interestingly, most versions of Wanner’s Hydra-Cell pumps are impervious to
inlet cavitation thanks to the Kel-Cell innovation.
Another cause of inlet cavitation is inadequate Net Positive Suction Head
(NPSH). Even pumps that are able to lift fluids have limitations as to how
much lift they can provide, so calculations are made to determine if the NPSHa
(available) is ≥ the NPSHr (required).
NPSHa = Pt + Hz - Hf - Ha – Pvp
Pt: Atmospheric pressure
Hz: Vertical distance from surface liquid to pumpcenterline (if liquid is below pump
centerline, then Hz is negative)
Hf: Friction loss in suction pumping
Ha: Acceleration head at pump suction
Pvp: Absolute vapor pressure of liquid at pumping
When using the formula above it should reflect the specific gravity of the
fluid and the units should be in “feet or meters absolute”.
The formula above seems “simple” but the formula for acceleration head is:
Ha = (L x V x N x C) ÷ (K x G)
L= Actual length of suction line (ft) —
not equivalent length
V = Velocity of liquid in suction line (ft/sec)
[V = GPM x (0.408 ÷ pipe ID2)]
N = RPM of crank shaft
C = Constant determined by type of pump
K = Constant to compensate for
compressibility of the fluid — use: 1.4 for de-aerated or hot water; 1.5
for most liquids; 2.5 for hydrocarbons with high compressibility
G = Gravitational constant (32.2 ft/sec2)
So you can see that the math gets complicated quickly, which is why we use
special spreadsheets to assist us these calculations.
Minimization of acceleration head and friction losses is achieved by
shortening the pump inlet line length (preferably ≤ 3 feet), using an inlet line
having an ID larger than the ID of the pump inlet and minimizing the number of
fittings used. An inlet velocity of 1-3 FPS (0.3 – 0.9 MPS) is ideal.
It is a good idea to cover feed tanks to prevent foreign material from
being introduced. It is also important to install a vortex breaker by the
outlet to prevent introduction of air into the pump inlet. The ID of the bulkhead fittings should match the ID of the discharge line
and their location on the tank should be above the bottom to reduce chances of
settled materials from being drawn into the pumps inlet port.
If your feed tank is used for more than one pump, it is advisable that
there are individual tank fittings for each pump system otherwise pumps
operating from a shared line simultaneously might “fight” each other for the
Non-air tight connections are another insidious way air enters your
system. This can be difficult to troubleshoot unless you are using clear tubing.
I recall one such application for a low flow rate metering pump that would
start-up fine, then after a short time would lose pumping volume. Luckily,
they were using clear tubing and we observed some air bubbles entering into the
line from a barbed elbow connection. The issue was solved once we
tightened the hose clamp.
Air was accumulating within a simplex pump chamber and preventing it from
fully filling with liquid, reducing the discharge rate. Always make sure
your connections are tight and realize that you might not observe leakage on the
inlet side of your plumbing but could still be sucking-in air. Helpful Accessories Minimizing the number of fittings and accessories contributes to
reliability, although there are certainly a few items that may enhance
performance or provide helpful information.
dampener installed on the inlet side of your pump system can reduce
acceleration head (Ha) losses and smooth-out a turbulent flow, which you
might expect from branches off a main or situations where one pump feeds
Pressure Gauges A compound pressure gauge to indicate vacuum and pressure on the inlet
side of the pump is an inexpensive way to confirm the suction condition.
If you are using filtration, a differential pressure gauge will provide a visual
indication of the pressure drop. These can be made with reed switches to
provide feedback to a control system or alarm. It is time to service your
filter once the NPSHa approaches the NPSHr or when the differential pressure
increases 5 PSI above the “clean” differential pressure.
Isolation valves keep fluid within the piping when the pump requires
servicing, so locating those to minimize product loss is a good idea.
Plumbing the pump with consideration of future maintenance or removal from the
system is a good idea also.
A pressure gauge located prior to a backpressure or pressure-regulating
valve enables accurate setting of those valves and reflects directly on the
If you have a batch style process that involves evacuating air from your
system for each start-up, using an automatic priming valve located near the pump
discharge port helps the pump prime quicker as there’s no need to move that air
downstream nor physical attendance by plant personnel, such as the case when
using a manually operated needle valve.
Well thought-out pump installations enhance performance and
serviceability; it’s an aspect we discuss with customers with each new
application based upon decades of in-field experience.
Chris Pasquali has
been trained by Wanner Engineering, having provided sales and engineering
support since 1991.