PUMP MAGAZINE: Questions and Answers (61-70)
Question #61 Dear Sir,
I am very much impressed with the kind of information
you are providing.
I want to know more about handling
abrasive & corrosive
liquids like Soap, Surfactants or Resins. I understand
that the running the pump at slow
speed would be a good option. Is it true
that Positive Displacement Lobe pump is a better
option? Can you throw more light on this? It would be very helpful for
me.
Thank you.
Regards,
Rakesh Gupta
Answer: Dear Rakesh,
Generally, Positive Displacement pumps are used for
viscous fluids (over approximately 500 cP viscosity), and centrifugals are
for low viscosity, although there is an overlapping
region, and occasional exceptions to
the rule. When abrasives are
present – things become more difficult, and wear
is the main issue. You are very much correct by thinking that slower
speed may help. As a rule of thumb,
the wear rate
is reduced at slower speed, and is a function of approximately RPM3 (cubed). This is why a larger size
pump, running slower, is often selected when solids are present.
Gear pumps traditionally
have not been a good selection for
the abrasive pumpages, because their bearings
(bushings) are typically in the pumpage
and are product-lubricated – thus wear
out fast if abrasives are present.
Gears are
used for two purposes: to pump the fluid (transfer it from
inlet to discharge ports), as well as to transmit
torque: a drive
gear actually turns the driven
(called idler).
Lobe pumps are close cousins of gear
pumps, but the difference is that
the lobes do not actually touch, and torque
transmission is done by another set of gears,
called timing gears, which are positioned outside pumpage, on the other side of the seals. Thus, the job of the lobes is
only to transfer fluid. Because if the fact that lobes do not touch
– they, in theory, last longer. Incidentally, the non-contacting nature of lobes is also the reason
why they are used for food applications, and lobe pumps are often FDA and 3-A approved.
However, their
bushings, just like in gear pumps, are product
lubricated, and will wear out, or
get plugged up by solids, similarly
to any other pump type that has
bushings in the pumpage.
A more common pump types, used for
solids handling are Progressing
Cavity or Peristaltic
(Hose), although Hose pumps may not be
good for sharp
solids (hose gets cut up), but very
good for softer
solids, even at very high concentration.
When you have a
combination of high viscosity and abrasives,
Progressing
Cavity pumps could be a good candidates, as long as the temperature is
reasonable (200-250 deg. F max.
usually). In your case, resins can be pumped well with PC pumps. The
disadvantage of PC pumps is size – they get to be rather long and take space, but if you have floor room,
then not an issue. As a rule of thumb, sizing of a PC pump is 75 psi per stage.
If you have a
specific application data, feel free
to enter it via Application Help
section of our Pump Magazine, and we
will assist with forwarding it to a qualified pump distributor.
For example, a Pumping Solutions
company, sells Allweiler Progressing
Cavity pumps, that have an added feature
of special stator design, allowing
higher differential
pressure,
thus reducing the overall pump size, and increasing
its reliability. We will forward your note to them for
information and feedback. They
handle US sales, as well as international
installations.
I hope this helps,
Dr. Lev Nelik, P.E.
Pumping Machinery
Question
#62: Hello,
Article #1
is touching on some interesting
points. I find special interest in
the second part - talking about recirculation
at low flow.
I will be grateful
for some leads to more information
explaining the mechanism of this phenomenon, it's maintenance aspects, ways to
discover and assure that this is the main cause.
I am a mechanical engineer working
at a Power Plant and currently trying to understand
and solve repeating frequent maintenance problems
(many years old) of Vertical Heater
Drain Pumps.
Sincerely,
Marchel
Answer: Dear Marchel,
-
We would be glad to help you with your problems,
as we often see similar issues at
power plants. One approach is to apply Simsite composite material design impeller,
with rings and bushings, - with hydraulics specifically fine-tuned for the current operating
conditions. You might have seen the description
of this approach in our recent Editorial article, as well as in About Us
general section.
To start,
could you send us some more information about your
pump: perhaps a sectional drawing, and a performance curve,
which should have H-Q information,
as well as NPSH, and efficiency lines. We will need these data to evaluate
design parameters, such as suction specific speed, recirculation
onset parameter,
etc., and come with correct hydraulics.
We could then help produce the
impeller in simsite composite material. With 80% lighter
then metal, rated to 400 deg. F (and
higher with special grades of simsite), and with tensile strength approaching
steel – you will have a much more reliable retrofitted pump. Simsite engineered
composites are significantly better then metals from
the cavitation standpoint, as well as handle to 15% abrasive
particulates. Plus, once we optimize
the hydraulics, you will see
significant efficiency improvements
– i.e. significant energy savings.
We apply this technique for
many power plants, as well as
chemical plants (excellent resistance
to chemicals), paper mills, and refineries.
Also, marine and navy pumps are another
examples of benefits of composites, since their
superior
resistance to salt water, brine
and brackish water, made them a material
of choice for water intake pumps, cooling and recirculating pumps, screen
wash, and similar.
Looking forward to hear
from you,
Best regards,
Dr. Lev
Nelik, P.E.
Pumping Machinery
Question #63 Dear
Sir,
Is it possible to operate
one pump in full flow and another
one partially using variable frequency
drive in parallel
operations?
Nagesh Kumar
NPC Company
Answer:
Two pumps running in parallel do not have to be identical, but the rules constructing
the resulting combined curve will still apply. When one of the pumps is controlled by the VFD, its head-capacity curve slides up or
down depending on the motor speed,
which is changed by the VFD. The
combined curve would need to be
constructed for
a series of such speeds, and the
intersection of each combined curve (at various
speeds) with the system curve will
give you the operating point.
You can find more information
on the parallel operation via SEARCH function. Some of the examples are Articles
#8, #13, etc. Pumping Machinery also
offers a consulting service to perform this, and similar
engineering requests.
We would need to get some more
details regarding
your applications to do that.
Regards,
Dr. Lev Nelik, P.E.
Pumping Machinery
Question #64 Dear Sir
I am
from a valve manufacturing company. I need to clarify
the following:
- Whether pre-heating
is required
for Austenitic Stainless
steels. During TIG it is
possible but during gas welding
I think without preheating it
is not possible to weld. Am I correct? Please explain.
- What is Sigma
Phase in steels?
- For CA15M (410)
casting, if I want to achieve 40 HRC - the heat treatment
practice as per standards
is Astonishing the steel to 100 deg Centigrade
followed by tempering at a
temperature
of 300-400 deg Centigrade. But
our supplier has given same astonishing temperature
but tempering temperature
of around 600 deg Centigrade to achieve the hardness
valve of 40 HRC. Is it correct?
- Will double tempering
reduce the hardness for
the above said material?
Thanks & Best Regards
S. Jayakanthan
Quality
Assurance
Dresser Valve
India Pvt Ltd
We
asked our contributor, Stephen Morrow, who is a Global Manager of Materials
Technology at ITT Industries to
comment:
1) You never preheat
an austenitic stainless steel - heating sensitizes it and temperature
controls are
aimed at minimizing heat effects.
2) Sigma phase is formed between temperature range
of 1050F and 1700F and is a brittle
phase of iron-chromium-nickel-molybdenum intermetallics
that reduce ductility and toughness
in a material. It is a brittle intermetallic
phase that forms in high chromium stainless alloys.
3) Heat treaters are responsible
for meeting hardness
requirements
- the correct
word is austenitizing. Hardness is controlled
by tempering after austentization to achieve desired final hardness.
4) Yes - double tempering will reduce
hardness. See ASTM A487 temper requirements for
CA6NM which calls for double temper at 1250F, then final temper
at 1100F.
As a suggestion, I
would recommend a reference
book on basic metallurgy. Good
start could be ASM handbooks, and a
copy of ASM Heat Treaters’ Guide for
heat treatment practices.
Stephen J. Morrow
Global Manager of Materials Technology
ITT Industries
Industrial Pump Group
Question #65 Dear Dr.
Pump
We are
using Smith and Loveless vacuum primed
centrifugal pumps in our influent lift station. We are
experiencing a problem with the vacuum priming
system water level. On pump startup
water enters
a sensing dome and totally fills it. On
pump shutdown this dome stays full of water. On the next pumping cycle, the vacuum pump
motors over
for a short
burst and draws
water into the vacuum systems
line. On each successive cycle water is drawn
further
up the lines until the total vacuum system is full.
In normal
operation the water would only rise
a small amount into the sensing dome since the sensor
probe protrudes down into the volute. On completion of pumping cycle the water would receed
into the volute. On next cycle this
would repeat. The water level would only increase
as the sensing probe became dirty and would not sense liquid at the bottom point
of the probe. Thus it gave you time in between senor cleanings.
This problem
has been occurring
since the last upgrade to this
station and the increase in pump
size and discharge quantity.
We have replaced
all fittings, tubing, valves and vacuum pumps with no success. Since these sensing probes
are located in the low pressure zone of the pumps, could these pumps be too large for
the application? We have been in touch
with the manufacturer and their
local repair
service has worked
on the system but neither has an
answer or
fix to the problem.
Thank you,
Robert
L. Challender, Sr.
From the Editor: The vacuum
primed applications are rather specialized field in the pump world. Editorial
Board had some trouble locating a qualified expert to assist with the answer
to Robert’s question. Surprisingly,
even well qualified pump professionals
could not comment on this item, due to apparent
unfamiliarity with this technology.
Yet, once we finally got the answer,
it became apparent that this method
can be applied effectively to many other
applications, even outside the traditional
waste treatment segment, if proper
familiarity and explanation of this
technology is presented to the users.
Our search finally ended right
where it should have started – at the Applications Department of Smith & Loveless Company, a manufacturer of
these systems. Dan Fisher and Karen Bowser,
both from Smith & Loveless not
only provided an answer and pointed to potential pitfalls, but they also provided a picture
of a system, with a brief
explanation of its operating principle, so that other
pump users would benefit from understanding
the concept.
Dan
Fisher and Karen
Bowser (Smith & Loveless Inc.)
comment:
Smith &
Loveless falls into the water/wastewater segment; this is our
sole area of concentration. However,
within that segment Smith & Loveless has enjoyed a strong
reputation with respect to its centrifugal
pump design and innovation. In addition to our
pump, our founders keenly recognized
the need for packaged lift stations
in wastewater systems, and thus
pioneered the factory-built pump station concept. Later developments included the above-grade pump station concept with vacuum-primed wastewater
pumps.
Let me offer a brief
overview of the kind of pump
(station) we have been discussing and the use of vacuum-priming
used in these applications. Mr.
Challender is referring to
what is called a Wet Well Mounted Pump Station, which is a lift station
containing two to four vertically mounted centrifugal,
solids-handling pumps (designed exclusively for
domestic sewage). The station base resides
above grade, mounted on top of the
sewage wet well.
All of the
pumps, controls, piping and valves
also reside above grade and outside of the corrosive wet well, maintaining a safe and clean environment for
routine maintenance and inspection.
The centrifugal wastewater pumps operate
by suction lift or what is called
"vacuum-priming".
The Vacuum Priming System is a simple process
that includes just three basic
components: a prime sensor, a solenoid valve, and a vacuum pump, with spare parts replacement kits similar
to shown below:
When the wet well level rises
to certain point, the pumps are automatically started
(Step 1). If the pump is not already
primed, it will require
the following two steps.
Step 2
When Step 1 occurs and the prime sensor
indicates the pump requires priming,
the vacuum pump comes on and the solenoid valve opens. The vacuum pump
evacuates air from the pump suction line and the pump through the 3/8" diameter
vacuum tubing. This causes wastewater
to fill the pump volute, cover the
seal faces and prime the pump.
Step 3
When the prime sensor indicates the pump is primed,
the solenoid valve closes, the vacuum pump shuts off and the pump turns on. This is all done in a few moments, simply
and reliably. From a totally non-primed
condition, the system is designed to prime
the pump in about 60 seconds under
standard rated
conditions. Once the pump is primed,
it is designed to stay primed
indefinitely.
We call it
vacuum priming because it uses a
small vacuum pump to assist in priming
the wastewater pump. Remember, this station sits several
feet above the fluid level and thus requires assistance when the pump needs to be primed. That's it. As such, the valve components do require a
simple periodic inspection or touch up cleaning.
We thank you
for contacting us and allowing us to
contribute to your online publication. For
more information
on above grade wastewater pump stations, we invite you to visit our Formula X(tm) Wet Well
Mounted Pump Station website.
Possible
reasons for
the encountered problem:
We don’t know the age or
the sizing of the pumps but there are certain
items that need to be checked. To start,
Mr. Challender
mentioned a recent upgrade to the pump. Assuming that they used Smith and
Loveless components, the upgrade
should not have caused the problem.
An increase in pressure
(TDH) could magnify an existing problem,
but again, should not be the source
of the problem.
Mr. Challender also mentioned that they had changed many of the
components, but have they simply tested for
a vacuum leak? An easy way to do this is apply a generous
amount of shaving cream to all of
the connection points while the vacuum pumps are
running. Any vacuum leak will be
immediately visible, and the connection can be easily repaired.
The next step would be to clean the electrode (sensor
probe), but we got the impression that they are
cleaning the electrode on a regular
basis. The only other component that
needs regular
attention is the solenoid valve. Any debris
in the seat of the solenoid may cause a leak. The stem of the valve needs to be
removed and the seat cleaned and
inspected. Any wear of the seat
would indicate a need to replace the
solenoid.
The next item is not a part
of routine maintenance, but could be
a solution. The electrode relay in the panel may be contributing
to the problem. Knowing the age of
this system would help, but we currently have a much improved
electrode relay.
That kit is available from the Smith
& Loveless Parts department. This department
can be contacted directly at
800-922-9048.
I hope that this is of some help, and I would invite Mr. Challender
to contact the factory directly if additional assistance is needed.
Dan Fisher and Karen Bowser
Smith & Loveless Inc.
Question (and comment) #66
These
comments and a question came from our reader in response
to a recent article
by Bob Hart (“Pump
Reliability – What Does this Term
Mean to You?”, Article #18 posted in
section Technical Articles). We will
post additional feedback from other readers in this section, as such feedback becomes
available to us. Let us hear your view on this important
subject.
Editor
Pump Magazine
Dear
Dr. Nelik
I thank you and Mr. Hart
for the subject well explained in Article #18
I also do agree
that the Equipment Reliability is responsibility
of multi-disciplined team which should make formalized
approach covering:
1) Selection of
equipment (effective design, maintainability, life cycle cost effectiveness)
2) Installation
3) Operational approach,
monitoring (parameters to be monitored,
how to monitor, and assessment of
values in view of reliability
followed by cost effective remedial
measure)
4) Maintenance -
applicable maintenance strategy,
benchmarking, inventory control
and all other associated activities.
To have an effective functioning of such team, a leader
is required.
My personal view is Maintenance
should be the leader. This is
because the definition of Maintenance, in my view, is: “A conjunction of
administrative and technical
actions, which are required
to revive an item or keep an item in a state at which it can provide desired
service as expected by user.
I would like you to comment on this.
Regards,
Sourav Kumar Chatterjee
Manager Rotating
Equipment
HPCL, Mumbai,
India
Question #67 Dear Sir,
The 33 TPh, 63 ata boiler is having a
Boiler feed pump of capacity 41 TPH and 92 ata. It is a directly coupled pump
to 200 kW Induction motor. The output is being throttled with a control valve
and for lower flow the water being recirculated through Deaerator. The pressure
drop across the pump is 25 ata. And there is minimum of 9 ata pressure
drop across the control valve due to valve design.
a) Providing Variable speed drive (VFD)
would be appropriate for the system to save energy? Whether the control valve
to be retained or suitably modified.
b) At lower load i.e. during start up the
water feeding to the boiler is low, hence recirculation would be
more. Will the VFD will take care of the for low flow requirement?
Dwajan, Power Plant, India
Answer: Dear Dwajan:
There
are two ways to reduce pump flow: either by throttling of a discharge valve, or
by changing a speed by a speed controller, such as VFD. The first method is
inefficient but simple. When changing a pump speed – flow, head and power
change in accordance with affinity laws: flow changes in direct proportion to
speed, head as a square of speed, and power as cube of speed.
You need
to construct a pump curve and a system curve, and see where these intersect –
that would be your operating point. System curve should account for friction
losses as well as static head. You should do that at several speeds to make
sure you always have enough pump head to overcome system resistance.
Take
a look at several other questions and articles that we have at various sections
within Pump Magazine using Search function.
If
you find that you are using less flow then required, and always bypass, then
even a VFD may not be the answer. In such case, you might be wasting a lot of
energy, and should consider a new impeller, designed for lower flow, so that
its BEP point is hydraulically shifted. We can provide such analysis for your
pump and system if you like.
Regards,
Dr.
Lev Nelik, P.E.
Pumping
Machinery
Question #68 Dr. Pump,
I just recently purchased an old cabin in
the mountains of PA. The well in which we get water is approx. two hundred feet
plus (200'+) deep. The pump that is there has the Goulds name on it and as near
as I can tell is what is known as a reciprocating pump. It has an electric
motor mounted on top of it (which may or may not be the way it was meant to
operate). There are varies lengths of wooden rods to get to water, each rod has
metal ends with male or female treads that are riveted to the wood. My question
is this can you tell me the era of this pump, its worth (it does work every
well), the operation of it, how to maintain it, and what I could
replace it with if I would need to? I thank you in advance for a quick and
timely response.
Stephanie Abel
Pennsylvania
Answer:
Dear Stephanie,
It sounds like you have a relic. Don’t laugh! – some
people actually collect really old pumps, and – in my own days at Goulds in Seneca
Falls – I recall we had an old Goulds pump mounted at the corporate lobby,
displaying it proudly to the visitors. Some of these old pumps are still
installed and work. I doubt, however, you will find any spare parts, and would
need to buy a more modern pump – from Goulds and any other pump company that
sells deep well pumps, - or work with your local mechanic that takes it as a
personal hobby to play with antiques. The wooden rods, for example, is clearly
something dating way back. You may want to note the serial number, pump model,
and any other information you may still have with a pump or any manuals that
miraculously might still be at the attic. (Unfortunately, sometimes it is only
possible when the pump is actually pulled up), and which time the installer is
already working on a new pump for you!).
What I suggest you do is this: look up a local pump
distributor in your Yellow Pages. Pump wells is a big thing in Pennsylvania,
and you should be able to find one easily, - there should be several listed for
your area. They will ask you the well size and how much water is there. They
will then quote you a deep well pump, with a motor. They normally also install
it. The whole thing should cost you, as a guess, between $500 and $1,000.
But do not let go your old pump! Goulds marketing
people might be willing to get it from you for their promotionals, and – the
next thing you know – you will inherit a small fortune from Goulds! You never
know: you might have “struck oil” – even with a water well!
Regards,
Dr. Lev Nelik, P.E.
Pumping Machinery
We have also obtained additional feedback from Goulds
Pumps Marketing group, Water Systems Division. George Strally kindly provided
this note:
Dear Stephanie,
The Goulds pump you describe is a very old "working head"
or "pump head" with a pump cylinder, a.k.a., a working barrel in the
water. The rods transmit energy to the
pump cylinder. Many of this style pump
were operated by windmills, tractor power take-offs and steam or gasoline
engines. They were produced from the late 1800's until the 1940's when they
were displaced by electric motor driven submersible pumps.
The pump cylinder has packings and check valves to allow
water into the cylinder and to keep water from flowing back into the well when
the piston is stroked up and down via the rods. This up and down movement of
the cylinder is a positive displacement pump, basically, each stroke of the
piston moves a volume of water up a distance equal to the volume (length) of
the stroke. We have had no parts available
for this style pump since the 1940's or maybe 1950's. I just completed 31 years of employment at
Goulds Pumps and I have seen them only in old catalogs and service
manuals. Unfortunately, we have no
electronic files for the old pumps which I can e-mail but I can mail or fax
copies of old catalog pages if you desire.
A model number or pattern number from the pump would be helpful, for a
200' well my guess is a 1454 or 1518 from a 1910 catalog.
I have no idea of its value.
If someone has a need for water in an area where there is no electric
service it would be more valuable than where there is power available. I have attached a brochure showing our most
popular 4" diameter submersible pump series, the GS as well as some
technical data and Installation manuals for submersibles and storage tanks.
They all open with Acrobat Reader. We suggest you contact a local Goulds Pumps
dealer through the Yellow Pages or website for first hand assistance. Goulds Pumps sells only through the
Professional Sales Channel. See: www.goulds.com. for a dealer locator and
product information.
Regards and Good Luck,
George Strally
ITT Goulds Pump, Water Systems Division
Question #69 Saludos Dr. Pump,
Me podria explicar el punto 5.1.10 de las normas API 610 referente al NPSHR.
PEDRO TOLEDO
Answer: Dear Pedro, -
Unfortunately my
Spanish is not good. I think your question relates to NPSHR as addressed by the
API specification. Which API Edition do you have? The latest is 9th,
although most people still use 8th.
Actually, the 10th Edition is being released as we speak.
I also recommend you
use SEARCH function on our web, and search for words like “NPSH”, “Cavitation”,
“Suction”, etc. – you may find many references and explanations that might
explain your question.
Again, sorry for my
Spanish language limitations.
Regards (Salutos!?),
Dr. Lev Nelik, P.E., Apics
Pumping Machinery
Question #70
I have a query on pump curves. We have a Sulzer multi-stage
centrifugal pump. I'm trying to determine the performance of the pump using the
pump curve provided by the manufacturer. Unfortunately, based on my
calculations, the pump head and the flow rate do not fit with the curve. I have
taken into consideration the velocity head, the friction losses and elevation head.
Formula used: Total Head = Total Discharge Head - Suction
Head.
Is there something else I'm missing out that is showing this
difference? In actual performance I'm getting 325 m3/hr but based on the curve
it should be about 375 m3/hr.
FYI, based on information 7 years ago and comparing the
discharge pressure and flow rate, I'm able to get this same numbers currently,
but how come with the calculation method I'm not able to do so. On the other
hand, with respect to instrumentation calibration, they are fairly accurate:
may be the pressure reading could be about 0.2 to 0.3 bar difference while the
flow rate could be off by about 10m3/hr.
Could it be that the curve provided by the manufacturer is
based on test conditions and could not be applied in actual conditions?
Look forward to hear from you.
Thanks,
Michael
Answer: Mike, - pump manufacturers usually test centrifugal
pumps on water, and this is what curves reflect. If you are pumping cold water,
you should get roughly the same results. Use our SEARCH function and type in a
key word such as "performance" or "curves" or
"head" etc. - there are many articles and discussion topics that
would pop up which have to do with definitions. Check these out first. If still
trouble, we may need to take a closer look at you curves, data, gage locations,
etc.
Regards,
Dr. Lev Nelik,
P.E., Apics
President
Pumping Machinery
Follow-up question: Dr. Nelik,
I would like to clarify for the condensate pump suction side,
would we need to consider the ambient pressure then start deducting off the
elbow friction loss, but add the static height and not forgetting to deduct the
vapour pressure. Here at the hot well/condenser it's a closed vessel. Is this
the right approach? At present I have not taken into account ambient pressure
at suction side.
Thanks,
Michael
Mike,-
NPSHA means net positive suction head available. It is
sometimes confusing. The word "net" is meant to imply the suction
head "above" the vapor pressure (expressed in feet (or meters if in
metric units)). What matters to the pump is what goes on right at the inlet.
Say you have a vessel with 20 feet of water above the impeller (fist stage if
vertical multistage pump) centerline. Say the vessel is open to atmosphere,
which is 14.7 psi or 34 feet. So - you have 20+34 = 54 feet suction head - so
far (we are not finished yet). Now - the flow flows from the tank to the pump.
Say the friction losses (elbows, bends, filters, valves, etc.) amount to 10
feet of hydraulic losses. Now you got 54 - 10 = 44 feet left. Next - what is
vapor pressure? For cold water, it is usually 0.34 psi, or about 0.8 feet.
So now: NPSHA = 44 - 0.8 = 43.2 feet.
If your tank is closed and under some vacuum, then the pressure
on its water surface is not 34 feet, but something less - say it is only 5
feet.
Then, NPSHA = 20 + 5 - 10 - 0.8 = 14.2 feet
Now, say your condensate is not cold, but at about 200
deg.F. At that temperature the vapor pressure is higher then for cold water.
Say the hot water vapor pressure is 4 psia (or about 9 feet) (I am just
guessing at numbers here, but you should use real values). Now you subtract
these 9 feet, which will leave you with less NPSHA then if it was cold water.
Also do not forget specific gravity (which may be less then 1.0 for hot water).
I hope this helps.
Regards,
Lev Nelik
Hello Dr. Nelik,
Thanks for the reply. Gets my understanding clearer.
Michael
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