PUMP MAGAZINE: Questions and Answers
(71-80)
Question #71 Dear Sir,
I enjoy the technical articles and Q&A sections, - they
are very informative and helpful.
Do you provide pump consulting? Is it strictly online, or can
you come to the field? We have a couple of problematic pumps, and would be
interested if you could help us solve these problems. How do we start?
Mike Sabert
Answer:
The first step is for you to email us a description of the problem. Try to include
as much information as possible so that we get a better picture. We will
evaluate and, if anything obvious, may suggest a solution right away, at no
charge. If the problem appears to be more involved, we may need to ask a few
follow-up questions, and will give you an estimate of the time and cost it
would take to fully assess your problem and solve it. We can often provide a
solution and recommendations interactively including a final report if
required, but if a field visit is required we can offer a field troubleshooting
approach.
Dr. Lev Nelik,
P.E.
Pumping Machinery
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Question #72 Dear Dr. Pump,
I just saw a question (#71) regarding your engineering consulting
and field assistance. We are a pump distributor, and occasionally have a need
for a technical support. We have our inside sales people who are good and know
the lines we handle, but occasionally the questions that our customers pose are
somewhat beyond our expertise, or too involved technically. We also have
questions beyond the immediate product lines we handle, and having someone like
yourself “on call” for such situations would be good for us and our customers.
In cases like these, we wonder if you might be interested to
be our “technical backing”?
Thanks,
John
Pump Distributor
Answer:
No problem. As you can see from our web site, the types of questions we get
involved with are technical, and require engineering insight. Some questions
can be answered right on the spot, and others may be more involved and require
calculations, drawings, engineering source books, lookup tables, etc.
We get questions
from direct pump users, as well as pump distributors, - from US as well as
worldwide. In most cases, our response is within 24 hours.
Let us know more
about your needs, your location, and some additional background. There are
several arrangements we can work out to provide the technical support you are
referring to, to extend and expand your technical muscle, by becoming an
addition or an extension of your Tech Support.
Regards,
Dr. Lev Nelik,
P.E.
President
Pumping Machinery
Question #73 Hi,
I found your site on the web during a pump search and am
quite impressed at the info you have compiled.
What I am looking for is info on "The Best" pump in
the World/USA for Agricultural/Nursery/Home use. The well was just drilled at
500 ft and I am looking for the most reliable pump on the market. Looking at
pumping about 50-70 gallons a minute from around 300-400 ft.
Thanks for any info you might have to help me.
Ron
CSS, Inc.
Answer:
Ron, - thanks for your kind words in reference to our web site.
We are forwarding
your question to our associate pump distributor, who is very competent with
pumps, including submersibles, - they will respond to you directly shortly. If
there is such “The Best Pump in the World” that you are looking for – they will
find it for you!
Best regards,
Dr. Lev Nelik,
P.E.
Pumping Machinery
Question #74 Dear Sir,
What is the effect on power when discharge head of positive
displacement pump/reciprocating pump is raised, while keeping the suction head
constant. What is the relationship of power with head? Kindly explain.
Thank you,
Dilip Mendhalkar
Answer:
Power is in direct proportion to the differential pressure. For example, in US
units it is:
BHP =
(ΔP x Q) / 1714 / EFF,
where (ΔP) is differential pressure in PSI
units; (Q) is flow in gallons per minute; (EFF) is pump efficiency; and 1714 is
a conversion coefficient.
Regards,
Pump Magazine
Question #75 Dear Dr. Pump,
What causes a fluid end to crack around the
suction valve seat deck for Triplex and Quintuplex
Plunger Pumps?
Bert Vicars
Answer: The area between the suction and discharge valves is subject to the
most fatigue stress. It cycles from zero psi to
discharge pressure with every rotation of the crankshaft. For a
conservative 300 RPM application that is 432,000 cycles per day. Most valves
are seated in the fluid end on a taper, so the higher the pressure the more the
suction valve is pressed down into the fluid end. This force, along with
the usual stress risers caused by corners near the suction valve, cause this
area to have the highest stress.
Andrew Shelton
Myers/Aplex
Company
Question #76 Dear Doctor:
After reading the articles published at your publication, I
would like to congratulate you for this kind of help for all who have interest
in learning more about centrifugal pumps. If possible, I would like to receive
more details about the term "RATED CAPACITY". What is the difference
between "RATED CAPACITY” and “NORMAL CAPACITY"? This concept is not
clear to me.
Thanks,
Agnaldo Borges, Mechanical Engineer
Answer: Dear Agnaldo, -
Use SEARCH
function of our web site, and type the word “API” – several answers will come
back. You should find some discussion there.
You are right –
this is confusing, and API-610 (American Petroleum Institute) should change
this definitions. This is why, in practice, nobody really applies this
definitions literally (because nobody really knows what this means!), but
instead elaborate a bit more when discussing. Usually, there is a condition at
which a pump is expected to operate – and it really (should!) be called “rated”
point. The word “normal” should really go away! There is little “normal” about a “typical” pump
operation! In general, a pump would almost never have the rated point exactly
where best efficiency point is. It is almost imposiible
to select a pump perfectly at the BEP: there is infinite number of operating
conditions, and only limited number of pump sizes to pick from. It would be
“nice” to “hit the BEP” all the time, but practically impossible. In real
world, a centrifugal pump operates all over the place, depending on a flow need
– and the “hope” is that operators will not operate the pump too far outside
recommended envelope of operation, which is typically between 70 to 120% of BEP
point, which is what API 610 calls for.
Hydraulic
Institute, which is sometimes a good source of information on mainly
definitions, unfortunately does not cover this subject sufficiently. API, which
traditionally has been more practical in this regard, has much more vigorous
approach, with more details. The reason API deals with a subject of limited
flow range is because it has a direct impact on reliability of equipment. API
committee members are practicing engineers, from the pump user community, and
form the pump manufacturers, while Hydraulic Institute has taken somewhat more
general approach, with not as much practically useful items, - although they
have published occasional useful material, such as pump sump guidelines, and
also a viscosity correction chart. HI has not been known, traditionally, as
having a strong focus on technical details
of engineering aspects of pumps.
So, why 70% is a
limit? What if a pump runs down to 69% BEP? Or 60%? Or 40%? How bad is it? - The further away from BEP a pump runs, the
trouble may happen. Radial thrust increases, suction recirculation
starts, shaft deflects, seal leaks, bearings get overloaded, etc. How bad is
bad? There are several related articles throughout the Pump Magazine, and you
can also look at those via SEARCH function and typing-in a keyword, such as “recirculation”, “thrust”, “cavitation”,
etc. In fact, there are formulas and more articles written on this subject,
where minimum flow relates to suction conditions, pump specific speed, pump
suction specific speed, pump energy level, etc., etc., and we would be glad to
help you with those if you like. However, for now, your next step is to do some
more SEARCHing by clicking a SEARCH button on our
entry page.
Thanks for asking!
– and keep on pumping!
Doctor Pump
Question #77 Dear Dr. Nelik,
I have been trying to find out what does a "STRING
TEST" mean for centrifugal pump train? Where can I find a good explanation
about this subject?
Thanks,
Agnaldo Borges, Mechanical Engineer
Answer: Dear Agnaldo, - you are
becoming our regular reader! Good for you! Keep learning!
A “String Test”
refers to a rotordynamic analysis of a complete
“train”, i.e. a pump, coupling, drives, flywheels, and anything else that is a
connected to a rotating “string” of rotors. There is a lateral and torsional analysis involved. For example, let’s say an
engineering company, such as Bechtel, Foster-Wheeler,
or similar is conducting an engineering analysis of such “train”. They would
obtain information on pump rotor mass and moment of inertia. They would get
similar data from a coupling manufacturer, motor manufacturer, etc. They will
combine these together into a “string” of computer data, and run a computer
program to calculate major critical speeds – usually the first 3-4 are most
important. Both lateral and torsional results are
produced, and if a “train” rotating speed is close to one of the critical
speeds determined by the analysis – there is a problem, and something then
needs to change. A change may mean increasing a rotor shaft, or decreasing its
length, or modifying a coupling design, or changing bearings with other types
having different stiffness and damping characteristics, etc.
In addition to
analysis, sometimes an actual field test is performed. However, the analysis
should be done first. Typically, this type of engineering work is rather
sophisticated, and is done for large units, with high energy density and
critical applications. For these, a field testing is to find a problem, for
example if vibrations are high and failures are frequent. A proper computer
analysis could avoid troubleshooting of a problem on a first place.
Pumping Machinery is
often involved in field troubleshooting. If you should have a specific need to
fix a problem with an installed piece of machinery, and a “string” is “not behaving”
– give us a call, we may be able to help.
Regards,
Dr. Lev Nelik,
Pumping Machinery
Question #78 Dear Doctor Pump,
I'm new in the industry and rotating machinery has become the
field which I have chosen to pursue. A simple (for you I think) question I
have: can or should a pump operate below its minimum impeller diameter? I ask
because I have new operating point for an existing (used) pump. Unfortunately
the new point is below the minimum impeller diameter as shown on the
performance curve.
Thanks.
Hope to receive your reply very soon!
Louie Eliscupides
Answer: Dear Louie:
There is not much
magic behind the minimum diameter. The main limitation is the impeller wear
ring diameter – at certain trim you would cut below the available metal, i.e.
the impeller will get worked out into a pile of machined chips! However,
strictly “hydraulically speaking” – you can extrapolate below the minimum, and
it will probably work fine, although obviously at poor efficiency (although
even that you can extrapolate from
the existing plotted curve that you have). Years ago, when I worked at
Ingersoll-Rand as a pump hydraulics designer, we tested impellers trimmed below
the minimum published diameter, with performance essentially following pump
affinity laws (subject to a so-called Stepanoff’s
correction factor for the diameter reduction below certain value, when affinity
laws begin to be less accurate).
From the logistics perspective, however, a pump
manufacturer usually will not guarantee a pump operation outside of their
published envelope (the less guarantees the better for them), so if you cut the
impeller below the published minimum, - you are on your own.
We invite other
people to comment. It would be of interest to hear the input of large and
reputable engineering construction firms, like Bechtel,
Fluor, and others, for example, what typical
practices, regarding impeller re-rates they encounter, in their interactions
between the pump users, and pump suppliers? I am sure our readers would be
interested to know, and a discussion could make an interesting topic and a good
contribution to the pumping community.
I hope it helps,-
let us know how your trimmed-below-the-minimum impeller works! By the way –
don’t forget to check out a Pump School section of our web site! – the next
Pump Training Event is coming up soon! – or, if you like, we can set up a group
training for your technical team at your own facility site? Let us know.
Dr. Lev Nelik, P.E.
Pumping Machinery
Question #79 Dear DrPump,
In one of the projects there are (2) boiler feed pumps of
capacity 190 m3/hr, with head of 1220 m, operating in parallel and feeding two
boilers through common suction and discharge header. The shut-off head
proposed by the consultant is minimum 125 % of rated head for better control
using feed control valve regulation since pumps are intended for parallel
operation. However, one of the Bidders offering the pumps for the project,
indicated that only 110% shut off head is sufficient for parallel
operation. Is 110% shut-off head is acceptable or a minimum of 125 %
shut-off head is preferable for the pumps in parallel operation?
Regards,
Krishna Kumar
Answer: API-610 spec (which is also often used by the boiler feed pump
manufacturers and users) 8th Edition states in paragraph 2.1.11 that
“Pumps that have stable head/capacity curves (continuous head rise to shutoff)
are preferred for all applications and are required when parallel operation is
specified. When parallel operation is specified, the head rise shall be at
least 10 percent of the head at rated capacity…”
The Bidder that
quoted 110% is thus technically within the acceptable range, although just at
the bare limit of it. As a note, for critical equipment, which boiler feed
pumps are, it may be a good idea for your engineers or whoever you use as
representatives, to actually witness the pumps being tested at the factory, and
not simply approve the curves after the fact. You want to make sure the testing
indeed produces a “continuously rising curve”, and not an “approximated” one.
You should specify in your purchase order witness test, so that the pump
manufacturer knows upfront this will be watched carefully. You representative
should be well qualified in pump testing, and should be standing right there,
next to the test engineer, actually taking the data along (pressures, flows,
power, temperature, etc.), and comparing his own plotted curve versus
manufacturers test department.
Regards,
Dr. Lev Nelik, P.E.
Pumping Machinery
Question #80
We often get inquiries from Paper Plants asking for pumps for
pumping 2 T/HR, at 4% pulp consistency. How do we convert this into M3/HR for
selection?
Regards
K. KANNAN
Answer: Fundamentally, a pulp mill produces pulp
stock, which is essentially a pulp (cellulosic
fibers) mixed (or suspended) in water. A mill’s objective is production of the
fibers, not water. The water is there only because a mill uses it as a medium
to transfer fibers. A mill would like, of course, to use as little water as
possible, but then the consistency of fibers would be so “thick” that the pumps
would not be able to pump it. Thus a compromise is found from experience. A
typical pulp stock (at the Pulp Mill) is 10 to15% consistency, and a typical
paper stock (which is actually used to make paper at the Paper Mill) is 0.5 to
2%. Some plants have a Pulp Mill and a Paper Mill at the same location, and
others have them separately, and transport pulp to a Paper Mill by trucks or some
other means.
Mills use various
terms to denote pulp consistency. A common term is “OD” (over dry), which is a
moisture-free fibers. If 100 kilograms of pulp stock contains 10 kilogram of
fibers, then the Oven Dry consistency is 10%, i.e. OD=10% .
So, the people who
produce fibers want to know how many kilograms (or pounds, if you are in a
non-metric country) of fibers they produce, but the people who pump this stock
want to know how many gallons per hour (gpm) or cubic
meters per hour (M3/hr), etc., will be flowing through the pump, so that they
can select the pump for the mill needs.
Thus the conversion
becomes rather straightforward. First, think how you would convert only water
from tons per day (TPD) to gallons per minute (GPM)? In US, 1 ton (called
“short ton”) is 2000 pounds. 1 gallon of water weighs 8.34 pounds. You can now
easily see that:
GMP(water) x 6 =
TPD(water)
(conversion coefficient
convenient come out as almost an exact number 6 for the US system of units)
But that is for water
only. If, only 4% is fibers, then multiplying by 4/100, we get the tons of fibers:
GPM(stock)
x 0.06 x C = TPD(“dried” pulp)
(where
C is stock consistency)
If your mill produces 2 tons per hour (2x24 = 48 tons per
day), of a 4% pulp, then back-solving for GPM we get:
Pumped stock flow
= 48 / 0.06 / 4 = 200 GPM
This means a pump must pump 200 gpm
of stock. You can convert to metric units by straight conversion:
200 / 4.4 = 45 m3/Hr
(approximately)
Keep in mind
proper units. Metric ton (I guess you would spell it as “tonne”?!)
is 1000 kilograms, but the US “short ton” is 2000 lbs (not the 1000 x 2.2 =
2200 pounds).
Also, paper mills
typically do not bother with the formulas, - they have charts where the
horizontal axis is gallons of stock per minute, vertical axis shows tons of
pulp per 24 hours, and there are lines of percent stock consistency on the
chart.
Let us know if
this helps,
Regards,
DoctorPump
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