Safety Factor Might Not Be Safe
Salam,
I know all of you Mechys and Civils here have seen this chart. It tells you a bunch of stuff. One of the most important stuff would be how flow, Q increases with the lowering of the head, H, and vice versa along the pump curve.
The power input to the pump, P2 increases with the
increase in Q.
If you oversize the H in the design, the actual H
during operation will be lower than the H at the red dot there. Oversizing H is
caused the fear that your pressure may be inadequate.
So you put in a high safety factor (SF) to total
pipe length (straight pipe length + equivalent length), static head difference
and residual pressure. But in actual operation, the H is much lower, (one of
the reasons being that the pipe is new) which causes the Q to increase (follow
the pump curve line to the right). See what happens to the power curve in the
chart below the pump curve.
There is an increase in P2.
In the post earlier, what happened during T&C was the rise of the Amps, due to higher Q than designed for.
Many would think that throttling the flow may
increase the resistance to the flow, thus increasing the H so that the Q is
brought down to the desired design point (the red dot).
True. Throttling is possible with the partial
closure of the valve downstream from the pump (if you're not sure what
'upstream' and 'downstream' means, please Google ya).
However, if you're having gate (or sluice) valve at
the downstream side (or discharge side) of the pump, you cannot do throttling
as the nature of the gate valve in such installation is for isolation only -
Yes - fully open and fully shut. But engineers do set the valve to close and
certain intervals, such as 20%, 50%, etc to see how Q changes with the added
resistance due to throttling, especially during T&C.
In operation, the valve is set to be fully open.
Gate valves are not meant to be throttled.
Throttling is only possible with Butterfly valves and Globe valves. These
valves are expensive compared to Gate. Reason for this is the design of the
valve which has a gate (Google or Bing for more info). If the valve is not
fully opened, the gate, in the long run may be damaged by the flow of water.
In that example, throttling was not the option. If
there's nothing that can be done at the site to lower the Amps, the only option
would be to remove the pump, send it back to the manufacturer to get the pump
impeller trimmed.
(I'm pretty sure you know what happens when the
Amps are constantly high, right?)
The purpose of trimming the impeller is to bring
back the pump curve to the actual duty point, which in this case the ACTUAL and
LOWER H, so that the Q won't rise so much which causes the Amps to spike.
This process is costly and time consuming. Imagine
if the development requires water supply in a month after the T&C of the
pumps and the pumps have to be removed for impeller trimming.
Wouldn't that be a nightmare? Just because the
designer has that fear factor and slaps in too high an SF in his/her design.
I'd like to repeat what I recommended in previous posts..
- Measure carefully the pipe length - Use
reasonable skill, care and diligence (this is when professional ethics
being applied)
- Count every valve and fitting possible from
your drawings to account for the fittings losses.
- Determine the static head difference
carefully. Try not to guess guess in the detailed design process.
- For residual head (the excess head when the
water reaches it's destination, for example an elevated storage tank or
reservoir), refer to standards, codes of practices or handbooks. For my
designs, I'd allow 4.5m.
- Add everything up and from that value, put in
an SF of 10%. There is no point allowing too high an SF if you have
measured and counted the elements in the pipeline carefully.
- Stop and check back thoroughly the
calculation.
There are designers who would allow SF in the pipe friction loss only. Some would add everything up and then apply that SF. What I would do is to consider the SF in the pipe friction loss only, which is different than what I've stated in item (5) above.
Be careful of design changes. If the design
changed, for example, the elevated storage tank is higher or lower than the
number you put in your design, it's better to revise it.
In normal practice, the contractor shall ask the
pump vendor to go thru the drawings and calculate the Q & H to determine
the correct pump size. Not because they don't trust you. They just want to make
sure that the pump is sized properly. Failing to do so and relying 100% on your
design - and if things messed up - touch wood, no one would be injured of
course but it's cost and time repercussions.
When the vendor submits to you the pump catalogue and curve for your approval, request for their pump sizing calculation and go thru it as - who knows, you might miss something in your calculations.
Our ACMV NSC is diligent. He prepares a very detailed chilled water and condenser water pump sizing calculation. This is important as the contractor already know the model of chiller, cooling tower, AHU and whatever else that's in the pipeline for him to calculate the pressure loss.
I'm writing this based on my little experience. I'm
sure that you may have a different way of doing the RIGHT thing and as always,
I welcome you to share your thoughts and experience for all of us to benefit
from.
Like I said before, I'm not Yoda. I don't know
everything there is to know about everything and I don't know what I don't know
either. But I don't have to become Yoda to share, ritht?
By the way, the pump curve above is from a pump manufacturer handbook. It's pretty good. Want a copy?
You know the drill - Please contact the friendly Vendor.
Oh yes by the way, Fire water pumps are different
animal. Those of you who know about stuffs like non-overriding etc, please
share your knowledge ya?
Till then, keep up the good work you all are doing.
Allah SWT knows best.
May we all be guided by Allah SWT, insha Allah. Amin.
Peace,
Wassalam,
FbI
Comments
Post a Comment