Bypass Valve problem 1

[Ings]

Hi,

I'm a engineer plant in a refinery.
There is a Control Valve 3" with a bypass control valve 4".

Last time the CV needed to be get out of operation to maintenance, so the bypass got in temporal control and a strong vibration started on the bypass line and valve...as a consecuence the Bypass valve lost its seal and i think another components where affected.

So, I want to studie the whole valve system, but I'm not sure about the normatives I need to be supported on in order to do that...i mean the calculation on the bypass valve in relation with the control valve...

I've red also about trims that reduces cavitation, noises, vibration in system. How can I identify, in base on my calculations, whether i need a trim whith special characteristics or not?

Thank you! Look forward for you comments

 

[gerhardl]

I am afraid you will have yo be a lot more specific to get concrete advice.

My suggestion is that you focus on the difference between the two valves. A larger bypass valve would suggest a larger capacity (trimtypes and sizes/cv for both valves will influence on real capacity and throtteling).

A larger capacity for bypass valve would then, at the same or lower throughlet as the original valve, require a larger degree of throtteling for the bypass than the original and would perhapse release a swinging regulation, or cavitation, or both.

Bends on bypass pipeline could induce more turbulent conditions. Placement of measuring points ( stable, not-turbulent measuring points required) could induce unstable regulating signals.

If the bypass valve is thought to have the same capacity and function as the riginal, and the original functions is OK, then the bypass should of course be as much equal as possible compared to the original.

 

[sheiko]

Ings,

My blind guess is that the bypass valve is oversized, but diagnose your problem, we will need more data like:
- the fluid? its state?
- the operating conditions? flowrates (min/norm/max)? upstream/downstream pressures? temperature?
- the valves (main and bypass) type? characteristics (linear? equal-percentage)? maximum Cv?
- Is the bypass manual?
- At wich opening are the vibrations starting?

Two remark: bypass valve is usually one size smaller than the control valve, and the bypass valve CV shall normally not exceed control valve CV...


"We don't believe things because they are true, things are true because we believe them."

 

[Ings]

HI, thank you everybody for your effort despite my lack of information.

The control valve (oil feed to heater)is installed in a pipe that has been reducted from 6" (principal head) to 4" and then for its installation to 3". It has no problem in control but its interns have presented slight deformation by vibrations, despite in data sheet design everything seems to be in order. (see it attached).

When the bypass valve was open (control valve out) this presented leak by its packing at the point that the heater has to be stopped.

This valve is a 4" globe, don´t have any information about it, but i estimate by the relation between the Cv and the Crane calculations ok K that Cv= 198.64. The bypass is a 4" pipe derivated from a 6" head pipe.

So how can I verify this bypass (including the valve) is well designed (not affected by noise or cavitation)? or otherwise this is a maintenance failure because a inappropiated packing gland adjustment?

This scenarie had happened in about three of the 8 valves of the two heaters, so is important to analyze this.

Hope you can help me...(sorry for my english mistakes)

 

[sheiko]

Ings,
We don't have any flowsheet to visualize the system, but if i have well understood, the main line and valve are 3"? whereas the bypass line is 6"?and the bypass valve is 4"? If so, this looks like a poor piping arrangement. Moreover, the bypass valve Cv you report is more than 2 times higher than the main valve rated Cv. This is a dangerous situation unless a PSV downstream can handle the bypass valve maximum capacity or a restriction orifice has been put on the bypass line to mitigate this flowrate.

"We don't believe things because they are true, things are true because we believe them."

 

[sheiko]

Bypass line is indeed 4". Sorry. But my remark is still the same.

"We don't believe things because they are true, things are true because we believe them."

 

[Ings]

I´m attaching the arrengement of the valve and its bypass

 

[sheiko]

Thank you. As far as i am concerned I can only comment on your files:
For example, your data sheet is not in line with the PID regarding the inlet and outlet line sizes (6" versus 4"). I have also noticed that, in your datasheet, the inlet pressure and the pressure drop are constant whatever the flowrate. Is that ok for you?

"We don't believe things because they are true, things are true because we believe them."

 

[Ings]

Well I`ve notice that in all the valves data sheets in the plant are the same, the outlet and inlet size considered are the one before and after the reductions.
And about the pressure and the pressure drop are constants, that`s right.

 

[stanier]

Is the bypass valve a globe valve? If it were in fact a gate or ball valve then the cavitation and vibration could be easily explained.

One way of reducing the cavitation would be to install a downstream multiple port orifice. Mitech and others make these. They also have software so you can model the system.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[Ings]

Yes, it´s a globe valve.

 

[sheiko]

Ings, you first description was: "...the bypass got in temporal control and a strong vibration started on the bypass line and valve...as a consecuence the Bypass valve lost its seal and i think another components where affected."

In my opinion, it is difficult to nail the problem as we don't have any information on your procedure for starting up the system with the manual bypass valve? like, what was the flowrate passing through the valve? At which opening of the bypass valve the system started to vibrate?, etc...
Just a wild guess, are you sure yuo haven`t created choked flow conditions? I believe that would explain part of your problems...

"We don't believe things because they are true, things are true because we believe them."

 

[sheiko]

In addition, the pipe size is critical in order to suspect vibration and movement – as well as the piping anchors.

"We don't believe things because they are true, things are true because we believe them."

 

[sheiko]

I have just came across this article:

http://www.chemicalprocessing.com/experts/answers/2007/073.html

It may help you.

"We don't believe things because they are true, things are true because we believe them."

 

[Ings]

Now the investigation of the occured has took placed, I got more information about what happened.
And apparently the operation was as normal as expected, the bypass valve was opening as the control valve was closing, taking an upstream flowmeter as a reference, the operator was keeping the flow constant until the control valve was closed.
On the field, the bypass valve had a leak as a told before, and this leak wasn´t controlable.
Now I can say that it´s more a maintenance issue, but because the investigation I need to demonstrate that the bypass it is well design.
Reding the article You Seiko post, I can see that as my by pass line is the same size that my control line (4" for the valve installation) it will be enough for keeping the plant working in case of a control valve failure.
But should I demonstrate this by calculations?

 

[bj45]

Ings, your problem surely is selection of manual bypass valve or a poor design for the function, pipe sizes look reasonable: 6"-4"-3" & 6"-4". Function of bypass could be for startup only, not for throtling in lieu of control valve. Vibration is due to unstable flow inside the valve, if not any other change occurs (cavitation, flashing, etc.). Bypass valve is for scrap now, internals are seriously damaged in minutes, leaky seats and poor stuffing box to sustain pressure when thottling.

 

[JimCasey]

The way I read the piping diagram, the bypass valve is manual. Manual globe valves are usually intended as isolation valves and the trim parts are "loose". The plug or disc is usually free to wobble a bit so it will find the seat, and sometimes free to spin on the stem so that closing the valve does not cause galling of the disc against the seat.

I agree the bypass valve sounds oversized.

What I believe is happening is that the bypass valve trim is going into an oscillation mode at very small opening. It's beating itself to death.
CONTROL valves are very stiffly guided to prevent this.

Oil can cavitate, but since it is actually composed of many compounds that just happen to boil within a particular range, the cavitation of oil is very gentle and non-damaging. Water is the culprit for cavitation damage as it is (1) pure (2) dense (3) releases a lot of energy as it condenses (4) has high surface tension.

Suggest you replace the bypass valves with valves identical to the control valves, possibly with manual actuators.