|Larry Bachus |
(a.k.a. Pump Guy)
To: Larry Bachus
Subject: Pumping Viscous Fluids
Larry “Pump Guy” Bachus,
I have some issues
regarding pumping of a viscous fluid. The fluid is a juice concentrate
with a reported viscosity of 15,000 CPS. Does a viscosity this high
The fluid is pumped with a positive-displacement pump from a
holding tank through stainless steel pipe. The pipe has a number of
bends, elbows, valves, and Ts. The fluid then passes through a meter
and into a mixing tank. When the holding tank level drops to a certain
point, the meter becomes inaccurate.
We suspect that air is being drawn through the system because of
the low suction pressure. But we are not sure.
We are trying to calculate the pressure drop in the suction
piping, but the numbers we have calculated using the normal method of
“equivalent lengths” do not make sense because of the viscosity. For
example, the Reynolds number we calculated is about 2.5, which goes off
the chart for locating the friction factor. I have two questions: 1.
What method can I use for calculating the pressure drop in the system?
2. Do you have any suggestions to resolve the problems we have at lower
From: Larry Bachus
RE: Pumping Viscous Fluids
A viscosity of
15,000 CPS (Centipoise) is completely logical for juice concentrate. I
tend to think of viscosity in the Centistokes Scale (CKS). Centipoise =
Centistokes x sp. gr. Toothpaste is rated at about 25,000 CKS. Peanut
butter is rated at about 350,000 CKS. (If I knew the sp.gr. of peanut
butter, I could calculate its viscosity in CPS). So, juice concentrate
at 15,000 CPS is perfectly logical. (If the juice concentrate has a
sp.gr. of two, then the CKS viscosity would be 7,500). This is about
right for juice concentrate.
Pipe friction tables (resistance and the Reynolds number) are
based on ambient water, not juice concentrate. The tables assume that
the pipe is new and free of scale. The tables also assume that all
valves are completely open with no resistance. The tables assume that
all filters and strainers are new and clean. The tables assume that the
construction and installation instructions were followed to the letter
with no deviation. None of this is reality.
|Resistance tables don’t reflect reality, and they often lead to pump problems.|
So, measuring the resistance with pressure gauges is the best way
to determine the actual resistance. Fill the piping system completely
with your juice concentrate and expel (burp) all air bubbles that might
accumulate in high spots and voids.
Install a pressure gauge at the beginning of a run of
pipe-and-fittings. This will be the upstream gauge. Install another
gauge at the end of the run of pipe and fittings. This is the
downstream gauge. Do this for both the suction side and discharge side
of your piping system. Note the differential between the upstream and
downstream gauge readings (suction and discharge) with the system off
and no movement through the pipe. If there is a differential, it will
reflect the different static liquid columns on the gauges.
Next, start your pump and allow the flow to begin and stabilize.
Now, note the differential pressure from the upstream to the downstream
gauges (suction and discharge) with the system functioning. The further
differential pressure between the two gauges will reflect the energy
lost into the system from velocity and friction. Convert this pressure
into head. This is the friction and velocity resistance across your
piping system. The resistance formula is called the “Bachus-Custodio
The tables are only a suggestion of possible friction. They are
moderately useful when the system only exists on the drawing board. But
once a plant is built and commissioned, the tables are useless. Why do
I say this?
Those tables don’t represent reality. Those tables don’t show the
resistance across a check valve with a weighted arm. Those tables can’t
record a hole, or plug, in a strainer basket. Those tables can’t record
the losses across a partially clogged filter screen. Those tables can’t
see a bolt or wrench stuck inside a pipe elbow. Those tables can’t
reflect a change or alteration that might occur at the construction and
installation phase. Those tables can’t measure the viscosity correction
factors. Those tables can’t measure any change in viscosity if the
liquid temperature changes. If you assume the fluid velocity from one
table and insert fictional numbers into the resistance formulas from
another table, then you calculate fictional friction values.
The gauges will record everything as it is now, including: the
wrench stuck inside the pipe elbow; the hole in the strainer basket;
the pipe joint gasket that interferes with flow: the check valve
flapper that won’t completely open; the globe valve that looks like a
gate valve; and the flowmeter Venturi that was inserted into the piping
last weekend that you don’t know about yet.
If the temperature, and thus viscosity, changes in the operation,
run the test at both temperature extremes. Now you can determine if the
suction piping arrangement is starving your pump, aspirating
air, or precipitating turbulence bubbles in the juice. Measure the
resistance losses with gauges. Don’t guesstimate the losses with
fictional numbers from those tables. This very discussion is in Chapter
8 (“System Curves and Troubleshooting Piping Systems”) of my pump book
(see page 42 for details). The exercise marches you step-by-step
through a realistic system.
That’s my suggestion. I employ the formula frequently to resolve problems with pumps. I hope this helps.
Larry Bachus (a.k.a. Pump Guy)
founder of pump services firm Bachus Company Inc., is a regular
contributor to Flow Control magazine. He is a pump consultant,
lecturer, and inventor based in Nashville, Tenn. Mr. Bachus is a member
of ASME and lectures in both English and Spanish. He can be reached at
or 615 361-7295.
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