undersized or Oversized Valve 2

[Gindi]

Hi

I am new to valve sizing and i have some question about which size to choose

Suppose the required Cv = 420, and the available sizes are as follows

NPS 6 --> Cv = 394
NPS 8 --> Cv = 567

if i choose the NPS 6 valve, is this considered as undersized valve? Does it have any other effect than increased pressure drop?

Is it the opposite for the NPS 8 valve?

I also have some general questions

if i decided to choose the smaller valve, which should add more pressure drop as i understand, is this considered good for valve authority?

Thanks in advance

 

[GAL02]

Dear Mr. Gindi, Good morning!
The Cv, or flow coefficient of the valve, plays a much larger part in the valve’s performance than determining how many GPM (gallons per minute) can pass through the valve. A valve that is sized correctly is a valve that is quiet, has accurate flow control, lasts beyond its expected life span, and does not leak. The Cv affects all of these things.

When a valve is too small for a given application you obviously won’t be able to get enough flow through the valve. However, what is often overlooked is that the system will be forced to compensate. Most engineers will oversize a pump by as much as 15% just to cover for mistakes like this. We don't see that as a reason to be lax when sizing your valve though. What actually occurs when you undersize a valve is the system compensates by raising the head pressure of the pump. At this point you’re going to have a much higher differential pressure across your valve. As the water leaves the valve, the sudden drop in pressure causes it to flash and bubble, otherwise known as cavitation. This is where the life of the valve comes in to play. The flashing of the water creates vibration and turbulence in the line, which over time will wear out the seats and seals in your valve. It can cause weak joints in the piping to leak; and the sound of the vibration can carry throughout your system.

If the valve is too large the system will have to be compensated with a balancing valve or something of the sort. With an oversized valve, the risk of damage is somewhat diminished. The real problem is controllability. Depending on how oversized the valve is, you may have to close it 50% or more to see any reduction in flow. This means that if you are modulating the valve your control loop will have to change drastically before you ever see a result. This will most likely cause your valve to hunt and overshoot constantly, which will wear out the valve and the actuator prematurely.
HAVE A NICE DAY.

WITH REGARDS

 

[gerhardl]

Hello Gindi,

GAL02 has covered nicely the importance of the total balance of a piping system, and consequences if largely undersized or overdimensioned valves.

Starting with your question with NPS6 and 8 valve/piping size and your Cv figures, the first thing is to confirm and check is the real data. Very often the figure given (from original source) for flow dimensioning is giving a false impression, and very often it will vary from very low to very high. Get at least the max, min and normal data. If large spread also get the dynamic picture: how long the min and max flow periodes are. Ref. to GAL02's comments.

Next, check flow speed in your system, and if this is within 'normal', acceptable limits for piping and valves. As commented, a higher speed will give a higher loss. This loss might be unwanted if the system has many units or if fed by pump, but might be OK for a natural fall system.

For your case and figures given: (sorry for the conversion to metric):

Cv's: 420, 394 and 567 G/min gives 31,30 and 42 l/s and for NPS6 (150mm) 1,8, 1,7 and 2,3 m/s. For NPS8 (200mm) the values are 0,99, 0,96 and 1,33 m/s.

Assuming water as an example you will get different opinions of what is 'normal'. Some will tell you (for the piping system, not necessarily single valves) that 2,33 m/s is too high, setting the limit at 2,0 m/s. Others will say OK to even higher speeds. At about 3 m/s (?) or higher you will get a wash-out speed used for cleaning lines.

Note: valves have flow speed limits either given by norms, or higher if specifically given (by written guarantee) by the producers!

Larger diameter lines will normally be dimensioned for lower speeds than smaller lines. NPS6 and 8 are in the 'smaller' range. In Europe there is a tendency to select higher speeds for modern equipment while US might be a bit more conservative.

 

[Gindi]

GAL02, gerhard

I am still confused with the concept of "I Won't get enough flow through the valve" for undersized valve.

When we say that, this means if the pressure drop through the valve is bigger than the pressure drop for chocked flow condition, i won't get the required flow. am i getting this right??

So basically, using the valve Cv i can calculate the pressure drop through the valve at the design flow rate, and if the pressure drop is less than the DP_chocked, then it is safe to use this valve.

Assuming the branch would have Balancing valve, shifting some of the pressure drop from the balancing valve to the control valve by reducing its size should be fine considering i won't reach the chocked flow condition. is this right?

To make it short: can i say that, the valve is considered undersized if it reached chocked flow condition?

Another part is the difference between the pipe size and valve size, if the valve size is 3-pipe-size or 4-pipe-size smaller than the pipe, is this OK? i realize that it might be refereed as bad pipe sizing, but lets assume that's what i have and i can't change it (For the purpose of learning) would this lead to a poor control or it won't effect the control?

For oversized valve, when shall i consider the valve to be oversized?

 

[georgeverghese]

One important case where a control valve is considered to be oversized is when the valve opening is less than say 5% of full stroke for the worst case identified turndown operation of the valve. In some case the upstream pressure at a control valve can be much higher than during normal operation during turndown.

 

[gerhardl]

Hello Gindi,

The best advice I can give you, is to forget the theoretical Cv calculations for the valves for a moment.

What are you needing the calculations for? Dimensioning a whole system with pipelines and valves in combination ar a single regulating issue for pressure and/or flow?

Valves are different, and equal types and nominal dimensions might have quite different borings and Cv's. For regulating, the boring and allowable flow and throttling, might be vastly different from the nominal pipeline data, varying from valvetype to valvetype and even from producer to producer. The reason for the different or missing answers to you is that there is no common mathematical answer covering all valves.

As previously mentioned, I would start getting the numbers for max, min and normal flow, and the dimensions for the pipeline from this.

Then I would put on paper the conditions (verbally, but with flow details (not Cv)) that the valves shall work with. If this is a pressure reducing or flow regulating issue I would describe the flow and pressure conditions acceptable (for and after). Then I would discuss possible solutions with an experienced supplier (or engineer) to be sure there is sensible commercial available and affordable solution out there.

And : There are always a vast selection of solutions and valvetypes, all very different.

Therefore: either verbally describe what you want to do and stop there, or select a valvetype and dimension and check if this covers your requirement for all conditions, without cavitation.

For an open commercial enquiry you use this as a basis, describing the valvetype as : 'for instance valvetype XXX for conditions (verbally expressed) or other types/variations if better suited'.

Some companies will force you to put anything on a valve data sheet with pure figures. Without written comments this will often lock the bidder to unsuitable solutions or confusion of what is actually wanted.

Best wishes!









Best wishes!

 

[pennpiper]

Gindi,
You wrote:"I am new to valve sizing and i have some question about which size to choose"

Did you ever consider using a line size valve?

Sometimes its possible to do all the right things and still get bad results