The following article by Bob Hart, a former Principal Consultant at DuPont Company, is presented here in response to a question posed by a reader of Pump Magazine, on the important subject of Net Positive Suction Head: required versus available. We have posted the reader’s question, and Bob Hart’s response, followed by this insightful technical article on the subject, for the benefits of the pump users. Additional related subjects can be found within the Pump Magazine site, via a Search function from the main site entry.
First, a
question by a pump user:
Dear Pump
Magazine,
How many times
NPSH-A can be higher than NPSH-R? As per CEP article (March 1993 page-81),
and Chemical Processing (December 1994, page 49), a higher
NPSH-Available is not good for the pump (?!). Is it true?
We have two
situations in the company I work:
(a)
pump operates (Ammonia) with 18.6 ksc(g) suction
pressure, temperature 37.8°C. It runs continuously without problems for over 25
years. The pump data sheet reads NPSHA = 20, and NPSHR = 11 feet.
(b) another pump operates with 8.46 Ksc
suction pressure, temperature 65.8°C(weak ammonium carbonate solution). This pump is also running without any
problems for about 10 years. The pump data sheet reads NPSHA is only 4.9 feet,
though the operating pressure is 8.46 Ksc. If I am
correct, the Head = P/Density = 8.46x14.22*/1.09/62.4 = 259 feet (1.09 sg)
Could you comment?
Thanks in advance,
Ramasami Anbazhagan,
a Pump User
Bob Hart comments:
For the
reader to fully understand the following comments, it must be understood the
term NPSH-Required, as defined by the pump industry, places the pump in a
degree of cavitation that is an unacceptable
operating condition. The more descriptive and appropriate term for this very
precise hydraulic condition that is now defined as the NPSH-Required of a pump
would be to define it as the
NPSH-Instability (NPSH-I) characteristic of the pump. All pumps will
experience a loss of head and many will become totally unstable (surge) when inlet hydraulic conditions (NPSH-Available) approaches the
NPSH-Required value.
The CEP
1993 and Chemical Processing 1994 articles referenced are not readily available
to this writer and hence the exact basis for the remarks may not be addressed
in the following comments. However, there are various articles that have been
written in the past few years that describe the cavitation
phenomena that point out, quite correctly, the rate of deterioration of the
surface on which the vapor bubbles implodes, is greater when the bubbles are
small than when they are large. The size of the vapor bubbles that will exist
within a pump is dependent on the margin between the terms NPSH-Available and
NPSH-Required. Extensive testing has been done to identify these phenomena on
specific pumps.
When
developing a pumping system, there are numerous decisions that must be made,
frequently without the benefit of knowing the exact pump that will be applied
to the system. These decisions frequently use the best information and judgment
available to the individual developing the system at the time with the end
result not having the advantage of laboratory test precision.
The
remarks in the inquiry would lead me to believe there may still be a
misunderstanding about the definition of NPSH-A as it relates to the Suction
Pressure of a pump. If the liquid being pumped has a very high Vapor Pressure,
the pump can have a high Suction Pressure but a very low NPSH-Available. The
NPSH-A may be less than the Submergence Level of the pump when considering the
friction losses in the piping system.
In
response to the specific question: ‘Is there a practical upper limit to the
Ratio of NPSH-A/NPSH-R?’, I would like to offer the
following:
My
industrial application experience has not identified maintenance, operating or
reliability problems that have limited plant production that could be directly
attributed to excessive NPSH-Available compared to the NPSH-Required. On the other hand, inadequate NPSH margin is
frequently a major contributing factor to these three parameters (maintenance,
operation, reliability) that limits plant production.
There
are a number of practical reasons that an excessive NPSH margin is normally not
applied to pump installations. Four major factors are:
Installation Costs
Increase with larger NPSH Margins
The Actual
NPSH-Available is typically less than calculated due to Fluid and Piping
conditions.
Manufacturer’s
NPSH-R Test Conditions are for new pumps
Hydraulically
stable fluids and piping systems seldom encountered in actual installations
Actual Operating
Conditions will typically vary significantly from the single point Data Sheet
information.
Robert
J. Hart, P.E.
******************************************************************************************
Technical Article
CAVITATION – A DANGEROUS ENEMY TO PUMPS
By
Robert J. Hart, P.E.
Robert J. Hart Enterprises, LLC
The term “Net Positive Suction Head – Required” applies to all pumps. It
is a term that has been – and continues to be – misunderstood by many of those
selecting pumps and designing the piping systems in which they are installed.
As a result, a number of pumps operate in varying degrees of cavitation, an underlying cause of the high maintenance
costs often associated with them. Cavitation, at its
worst, sounds like loose gravel passing through the pump. Damage to seals,
bearings and impellers will usually be experienced well before the noise of cavitation can be detected by the human ear.
The values of NPSH-Required, published in most manufacturers’
Generalized Performance Curves, are values that place the pump in controlled,
but heavy cavitation. The NPSH-Required is defined as
the NPSH applied to the pump at a given flow rate which causes sufficient cavitation to reduce the Total Dynamic Head (TDH) by 3%.
This is an established pump industry standard procedure used to indirectly
measure, at a reasonable cost, the suction side pressure loss inside a pump
before mechanical action increases the liquid pressure. The measurement is
taken while pumping water with a minimum of inlet stream turbulence (i.e. no
close connected, double elbows), no entrained gas, and frequently, with water
that has most dissolved gas removed. These factors will increase the
NPSH-Required values obtained during the test. While this may appear to be an
idealistic system, and is not realistic for actual operating conditions, it is
the only method that can provide reproducible test results. The user must apply
a margin between the liquid in let total pressure and its vapor pressure
greater than this pressure loss to prevent vapor formation at the impeller
inlet.
Even though Figure 1 illustrates the typical vendors’ published non-cavitating Total Dynamic Head curves (head vs. Flow) and
the NPSH-Required curve (NPSH-Required vs. Flow) on the same graph, it must be
understood the NPSH-R values applied to the pump will reduced the Total Dynamic
Head developed by the pump by 3% at any flow rate. While the hydraulic loss due
to reduced TDH is typically not significant, the resulting shock on the
equipment can reduce the mechanical life of the seal, bearings, and impellers.
Figure 1 Published generalized performance curve
compared to TDH with NPSH-A/NPSH-R = 1.0
NPSH-Available
Manufacturers expect the user to supply an NPSH-Available which exceeds
the NPSH-Required value that is published. The margin between the NPSH-Required
becomes a commercial decision and should be properly evaluated by the person
selecting the pump and developing the piping system. Systems which are pumping
liquids at their equilibrium condition, i.e. ready to flash with an increase in
temperature or a decrease in pressure, require special consideration when pump
is being selected and the system is being designed. The NPSH-Available in such
systems normally can be significantly increas3d only by:
increasing
the supply tank elevation above the pump;
lowering
the pump relative to the supply vessel;
providing
a booster pump;
cooling
the liquid to reduce the vapor pressure characteristic.
PUMP APPLICATION CONSIDERATIONS
As a guideline, the NPSH-Available should exceed the NPSH-Required by a
minimum of 5 feet, or be equal to 1.35 times the NPSH-Required, whichever is
the greater value. As an example, for an NPSH-Required of 20 feet, the
NPSH-Available should be a minimum of 27 feet. This is considered the minimum
acceptable margin, and even then, some degree of mechanical and erosion damage
can be experienced. Increasing the margin will improve the reliability and
acceptable operating range of the pump. In order to totally eliminate cavitation, which affects the Total Dynamic Head, the Net
Positive Suction Head-Available must be from two to five tomes the
NPSH-Required, depending on the operating flow rate relative to the Best
Efficiency Point flow rate of the pump design, as Figure 2 illustrates.
Figure 2 Mechanical damage
requires little elapse time (hours to weeks). Erosion damage requires longer
elapse time (days to months)
Pumps that operate with NPSH-A/NPSH-R ratio above 1.35, but below the
2X to 5X parameter, can have what is considered “acceptable” (but not optimal)
seal and bearing life; however, they may be vulnerable to erosion damage to the
impeller, which will require more frequent impeller replacement than would
otherwise be experienced had the cavitation been
totally eliminated.
Anyone developing pumping systems designed with these minimum
recommended margins should consider testing the equipment to be delivered to
confirm that it meets the published data. The following information should be
considered when ordering the pumps to decide if tests should be conducted:
Most
manufacturers do not hydraulically test pumps unless required to do sop by the
purchaser
The first
test5 of a specific pump may result in higher NPSH-Required and Total Dynamic
Head values than indicated by the published data due to casting variations of
the casing and impeller and the manufacturer’s documentation to test
procedures. To correct this condition, the manufacturer may have to grind the
controlling surfaces of the impeller and case to reduce the NPSH-Required and
reduce the impeller diameter to reduce the TDH values within acceptable
tolerances. After such modifications are made, the pump is retested to confirm
the result of the rework.
The
testing procedures followed by most manufacturers (per hydraulic Institute
Standards) typically yield the minimum information, which is not considered
adequate for most critical services, especially as it relates to NPSH-Required
testing. Recommended test procedures, the critique of proposed test procedures
and/or witnessing of critical equipment tests can be supplied on requests.
In today’s business environments, plant investment must reap the maximum
return. It is advisable that those making decisions regarding new pumping
systems or troubleshooting existing systems take heed of this information.
Assistance on thee topics can be supplied on request.
Note: Bob Hart has spent 27 years as a Principal Consult in the
Rotating Machinery Group with the Engineering Department of the DuPont Company, Wilmington, DE