14 replies to this topic

[hirengdesai]

Dear all,

This is regarding requirement of single/restricted orifices installation in the emergency depressurizing line.

During detail engineering of Hydrogen plant, we faced this problem.

In RO specification sheet issued by Process engineer, he has specified cerain flow required to be relieved (say 25000 nm3/hr) with P1 and p2 as upstream and downstream pressure respectivey.

Here, P1 and P2 vales were 30 bar a and 1.5 bar a respectively. RO is placed in the emergency depressurization line connected to flare header.

However, while selecting orifice for this service, our Instrument engineer faced difficulty in maintaing both criteria, i.e. Pressure drop and flow.
As per INstrument engineer,multiple orifice plates are required to meet both these criteria.

If single orifice plate is used, then flow requirement can be meet, but the pressure at the downstream of RO would be higher than P2 (Choked flow will occur at pressure higher than p2).
He was of the opinion that,as the RO is connected with flare header at 1.5 bar pressure, the fluid will immediately see that pressure only, so single orifice plate should also be suffice if sudden pressure reduction at RO outlet is not a concern.

My query is as follows:

For cases as mentioned above, which is preferred as per standard engineering practices, single orifice plate installation or multiple orifice plates installation ?

Thanks and regards,
Hiren Desai

[daryon]

Hi,

I agree with the process engineer. The pressure downstrem P2 will be whatever pressure in the flare header is at the time of depressurisation plus whatever back pressure is built up in the discharge piping downstream of the RO. The fact the flow is choked will not affect the downstream pressure, what choked flow means is that when the actual pressure drop across the orifce exceeds the value that results in choked flow (gas sonic velocity is achieved)any further decrease in downstream pressure will not increase the mass flowrate through the orifice.

So what is likely to happen in your scenario is that choked flow occurs at say half the avialble pressure drop, 14 bar and you will get the same mass flowrate through the orifce when the pressure drop across the orifice is 29 bar. I do not think the pressure drops you are talking about warrant multiple RO, you can easily get an RO that can cope with 29 bar pressure drop. No problem

Edited by daryon, 09 September 2009 - 12:17 AM.

[hirengdesai]

Thanks Daryon,
so what i understand that RO will kill the pressure upto the flare header back pressure and at the same time deliver the flow which will be mostly at sonic velocity.
hence, multiple RO installement in such cases are not required.
pl correct me if i am wrong.

Thanks again,
Hiren

[JoeWong]

Hiren,

High pressure drop across RO lead to high noise level and vibration. This may lead to downstream piping failure. One of the measures is to provide multiple RO in series to drop the vibration level and avoid potential failure of piping.

[shan]

If the back pressure caused by the release is ignored, P2 is always 1.5 bar. The flow depends on P1 and RO area if choke flow conditions is satisfied. One RO is OK for P1 @ 30 bar.

[hirengdesai]

Thks guys for your reply.

i) As Joe Wong has rightly said, vibration and high noise level could be a problem.
However, i request him to elaborate more regarding "failure of downstream piping", because as such velocity (Mach number) in the downstream tail pipe would reduce because of increase in the Area. Is it because of conversion of kinetic energy in the downstream section ?

ii))Do we need to consider failure of RO itself ?

iii) Another thing i would like to ask is regarding allowable maximum pressure drop through an orifice . Is there any guideline for this ?
Can RO be specified for more than 50% pressure reduction?

Please advise.

[daryon]

Hi again,

i) High pressure drops and high gas velocities can generate acoustic fields within the pipe. This can result in high-frequency, large amplitude pipe vibration, putting at risk the pipe wall and branch connections. In some applications (blowdown lines and PSV discharge pipes) the level of vibration can induce fatige cracking of the pipe wall leading to failure. There are companies that specialise in AIV (acoustic induced vibration) studies. I have expericence of SVT engineer consultants and they are a good outfit. The studies are expensive and are only really necessary on process lines within a plant that have very large pressure drops and high velocities. Your pressure drop of 30 bar i don't think warrants this.

ii) Failure of RO is unlikely, they are constructed with a thickness that can withstand the pressure differential for a given bore diameter

iii) The plate thickness of the orifice will be larger for higher pressure drop. I had a quick look at a DS on a previos job for a pressure drop of 100 bar the plate thickness was roughly 10 mm for a 17.5 mm bore diameter. An RO is normally specified with a pressure drop not a percentage of the inlet pressure.

[hirengdesai]

Thks Daryon for your detailed explanation.


As per you and Shan, 30 bar pressure drop will not warrent multiple RO. However, the question i still want to ask is :

Up to what maximum pressure drop, single orifice installation are specified normally [considering the facts such as vibration problems, RO plate material limitations, (if any), etc.] or it is system specific and there is no general guideline for this ?


warm regards,

[shan]

Thks Daryon for your detailed explanation.


As per you and Shan, 30 bar pressure drop will not warrent multiple RO. However, the question i still want to ask is :

Up to what maximum pressure drop, single orifice installation are specified normally [considering the facts such as vibration problems, RO plate material limitations, (if any), etc.] or it is system specific and there is no general guideline for this ?


warm regards,

I have never specified a double orifices blowdown line even if release pressure is 2000 psig.
There may be liquid condensed because of J-T through orifice. How can you handle the liquid accumulated between the first RO and the second RO if double ROs are installed?

[JoeWong]

daryon has rightly explained the Acoustically Induced Vibration (AIV) fatigue phenomenon. This phenomenon is based on simple energy conversion and transmission. As fluid passing through a restriction i.e. control valve, restriction orifice, tube, pressure relief valve, etc, kinetic energy will be converted to acoustic energy. Acoustic energy will be transmitted to downstream piping in the form of vibration. High frequency with effective vibration amplitude would lead to piping fatigue failure. More discussion in "Acoustic Induced Vibration (AIV) Fatigue", "Piping Excitation When Expose to Acoustic Energy" and "Extra Attention to Common Point and Similarity on AIV Failure"

AIV largely depend on flowrate and pressure drop. Low pressure drop i.e. 30 bar may have low chance result AIV fatigue problem, however it is not 100% warrants problem free. Calculation of Sound Power Level (PWL) and check against piping PWL fatigue threshold. A few methods i.e. D-method, D/t-method, E-method, etc may be used.

ii) RO vendor will responsible to calculate RO thickness sufficient withstand the stress due to flow.

iii) As per my experience, RO can take any pressure drop, subject to RO thickness. But i am sure, there will be some limitation which i am not aware of. Hope some instrument engineers can come in to confirm my statement.

[hollerg]

1) The design of the downstream pipe needs to ensure that the choke point is indeed the orifice, otherwise the flow may not be as you expect. This typically means the piping downstream is taking far less pressure drop.

2) Usually in the back of Hydrocarbon Processing or Chemical Engineering magazines you can find an ad for controling the sound from choke flow through a valve. They might even design an orifice plate pack to both control flow and attenuate the sound.

Good Engineering

[Himanshu Sharma]

Warm greetings to all !

Though the topic is been elaborated beautifully by experts still i would like to add my two cents.

Problem is imposed by choked flow , an ancient rule of thumb says that if ratio of Upstream and downstream pressure is greater than 2 there are good chances that flow is choked downstream of orifice that's why we go in steps for pressure reduction.one step reducing 50% pressure and continuing so forth.

Even if the orifice plate flow characteristic can assure you the flow which is needed in actual the flow will be much lesser as driving force will be Upstream Pressure minus choked Pressure .Flow restriction is not only problem there will be a sudden surge and vibration in piping system that may be dangerous.

There is one more interesting fact that as per thermodynamics only velocity is choked not he flow as we commonly refer ,with the increase/decrease in pressure upstream side mass flow through orifice will vary because of change in density.

[JoeWong]

Glad that you learnt some ideas here...

[shan]

Warm greetings to all !

Though the topic is been elaborated beautifully by experts still i would like to add my two cents.

Problem is imposed by choked flow , an ancient rule of thumb says that if ratio of Upstream and downstream pressure is greater than 2 there are good chances that flow is choked downstream of orifice that's why we go in steps for pressure reduction.one step reducing 50% pressure and continuing so forth.

Even if the orifice plate flow characteristic can assure you the flow which is needed in actual the flow will be much lesser as driving force will be Upstream Pressure minus choked Pressure .Flow restriction is not only problem there will be a sudden surge and vibration in piping system that may be dangerous.

There is one more interesting fact that as per thermodynamics only velocity is choked not he flow as we commonly refer ,with the increase/decrease in pressure upstream side mass flow through orifice will vary because of change in density.

Thank you for your warm greeting. However, you are totally lost.
The orifice on the blowdown line is not for choked flow. It is for flow rate restriction to satisfy flare capacity limitation. Let us calculate how many stages of RO needed for a 30 bar upstream line if 50% pressure drop each step and continuing so forth to reach 0 bar. The first step is from 30 bar to 15 bar, then 7.5 then 3.75 bar, 1.875 bar… It will be never reach 0 bar. Therefore, it is a project of your life time if your boss has the same concept as yours.

[ARAZA]

Hi There,

I recall a case where there had been a similar incident of pipe induced vibrations and cracking of the pipe itself due to higher velocities downstream of the control valve. It was found that noise and vibrations are due to high velocities and the Mach number calculated was 0.7, the problem was rectified by installing a larger size control valve. After installation, the vibration problem was solved and the Mach Number calculated was 0.3.

The orifice plate is analogous to a control valve and is intended to drop the pressure.

How much diffrential pressure you can bear?? IEC had established a guideline of 0.3 Mach. Keep noise and vibration problem at bay by designing the system with a velocity such that Mach number is 0.3 or less. This might take more than one orifice plates to achieve the desired pressure.

For further reference, check out IEC 534-8-3. This explains the allowable noise from a control valve or from a pressure reducing devices.

Regards,

Araza