28 replies to this topic

[Propacket]

Hi,
I need to size a blowdown valve in a gas processing plant.I have a searched a lot but i could not find even basic literature for this.Can anybody help me get the literature or advise me the procedure for sizing.

[daryon]

Hi,
I need to size a blowdown valve in a gas processing plant.I have a searched a lot but i could not find even basic literature for this.Can anybody help me get the literature or advise me the procedure for sizing.

Hi,

Typically gas processing equipment will be blowndown through a restriction orifice (RO) which is located downstream of a actuated valve. The actuated valve fully opens when blowdown is intiated and the RO controls the rate at which the gas processing equipment is blowndown.

When sizing the RO you have to consider how quickly you need to reduce the pressure in the gas processing equipment by releasing gas into the flare system. API has some guidence on this:

"…provide depressurising on all equipment that process light hydrocarbons and set the depressured rate to achieve 100 psig (690kPag) or 50% of the vessel design pressure, which ever is lower in 15 minutes."

The larger the RO bore diameter the faster the pressure reduction. You also need to think about much flow the falre system can handle. If the RO is sized with too large a bore diameter then a large flowrate can pass through it and you may potenitally exceed the capacity of the flare system. You need to look at the flare system capacity and which parts of the system are going to be blowndown simultaneously, and make sure your peak blowdown rates do not exceed flare capacity.

As for the actual RO bore diameter sizing calculation this is best done using HYSYS depressuring facility (or similar) if avialable. These calculations are dynamic in nature because as soon as blowndown is initated (t=0) the system upstream of the RO is constantly changing. The pressure is reducing which is changing the gas properties and also the driving force to push the gas through the RO (i.e the flow through the RO starts high and reduces with time).

You can do hand calculations (approximations) and break the calculation into time steps, so a t=0 you calculate how much gas will pass through a given RO size, then at t=1min you know how much gas has left (assuming gas flow rate is constant for a minute... which it won't be hence the approximation) so you can calculate a new pressure and gas properties. The you and work out how much gas will pass through the orifice until t=2min given the new upstream conditions, and just keep repeating the procedure. If you get to t=20mins and the pressure in the system is too high (>690 kPag) you know your RO is too small. Selct a large RO diameter and re-do the calculation. The smaller the time step the more accurate the caculation will be. The depressurising utility in HYSYS allows you take very small time steps (0.5 sec) and complete the blowndown calculation very quickly, hence it is highly recomended.

hope this helps you

[JoeWong]

It is great that daryon has provided you with some good points...

I just want to highlight a potential confusion in concept...

...You need to look at the flare system capacity and which parts of the system are going to be blowndown simultaneously, and make sure your peak blowdown rates do not exceed flare capacity...

I just copied over what has been written earlier in "Don't misunderstood depressuring".

The main purpose of depressuring is to evacuate the inventory from process system as fast as possible so that the reduced internal pressure stresses is kept below the rupture stress. This has been discussed in "Depressuring within 15 minutes no longer applicable ?". Nevertheless, quick depressuring may lead to other problem such as low temperature embrittlement, excessive noise and vibration, etc. Depressure a high pressure would lead to low temperature of depressured system and failure due to low temperature embrittlement. Higher the depressuring rate, lower the temperature can be experienced by depressured system. Thus, the RO downstream of BDV in depressuring system primarily is to limit flow so that the temperature will not drop below the allowable lowest temperature limit of material.

For some system (generally low pressure), quick depressuring may not be required as the internal pressure deduced stress may still well below the maximum allowable stress of depressured system. Under this scenario, the RO is to limit the flow in order to minimise disposal and flaring capacity (this pretty inline to what the young engineer explained to me).

[Propacket]

Thanks to all of you. I have learnt so much from your replies.I have tried depressuring utility in hysys.It gives excellent result.Thanks again.

Hi,
I need to size a blowdown valve in a gas processing plant.I have a searched a lot but i could not find even basic literature for this.Can anybody help me get the literature or advise me the procedure for sizing.

Hi,

Typically gas processing equipment will be blowndown through a restriction orifice (RO) which is located downstream of a actuated valve. The actuated valve fully opens when blowdown is intiated and the RO controls the rate at which the gas processing equipment is blowndown.

When sizing the RO you have to consider how quickly you need to reduce the pressure in the gas processing equipment by releasing gas into the flare system. API has some guidence on this:

"…provide depressurising on all equipment that process light hydrocarbons and set the depressured rate to achieve 100 psig (690kPag) or 50% of the vessel design pressure, which ever is lower in 15 minutes."

You can do hand calculations (approximations) and break the calculation into time steps, so a t=0 you calculate how much gas will pass through a given RO size, then at t=1min you know how much gas has left (assuming gas flow rate is constant for a minute... which it won't be hence the approximation) so you can calculate a new pressure and gas properties. The you and work out how much gas will pass through the orifice until t=2min given the new upstream conditions, and just keep repeating the procedure. If you get to t=20mins and the pressure in the system is too high (>690 kPag) you know your RO is too small. Selct a large RO diameter and re-do the calculation. The smaller the time step the more accurate the caculation will be. The depressurising utility in HYSYS allows you take very small time steps (0.5 sec) and complete the blowndown calculation very quickly, hence it is highly recomended.

hope this helps you

Sorry to jump into this thread, but I was wondering what equations do i use for the hand calc. I have got the Grote equation but I would like to understand how this equation is derived. I am looking to create the blowdown profile for a non-controlled depressurisation.

Any help will be greatly appreciated!

Ken

[JoeWong]

Sorry to jump into this thread, but I was wondering what equations do i use for the hand calc. I have got the Grote equation but I would like to understand how this equation is derived. I am looking to create the blowdown profile for a non-controlled depressurisation.

Any help will be greatly appreciated!

Ken

I have previously tried the derivation of Grote equation. It typically derive from
(i) mass flow through an orifice
(ii) mass change rate in a system

Please take note derivation based on :
i) Critical flow throughout entire depressuring process
ii) Constant mass flow throughout entire depressuring process
iii) System being depressured is maintained as gaseous throughout entire depressuring process
iv) Constant temperature, molecular weight and compressibility

If your system contain liquid or possible liquid formation along the depressuring, etc, then deviation will start...

Check out Depressuring Flow - Quick Manual Method. It is always advisable to refer to original Grote article...

[Propacket]

Sorry to jump into this thread, but I was wondering what equations do i use for the hand calc. I have got the Grote equation but I would like to understand how this equation is derived. I am looking to create the blowdown profile for a non-controlled depressurisation.

Any help will be greatly appreciated!

Ken

I have previously tried the derivation of Grote equation. It typically derive from
(i) mass flow through an orifice
(ii) mass change rate in a system

Please take note derivation based on :
i) Critical flow throughout entire depressuring process
ii) Constant mass flow throughout entire depressuring process
iii) System being depressured is maintained as gaseous throughout entire depressuring process
iv) Constant temperature, molecular weight and compressibility

If your system contain liquid or possible liquid formation along the depressuring, etc, then deviation will start...

Check out Depressuring Flow - Quick Manual Method. It is always advisable to refer to original Grote article...

You said that "System being depressured is maintained as gaseous throughout entire depressuring process". But hysys depressuring utility sizes blowdown valve for both gas and liquid. What does it mean?

[JoeWong]

You said that "System being depressured is maintained as gaseous throughout entire depressuring process". But hysys depressuring utility sizes blowdown valve for both gas and liquid. What does it mean?

Grote equation only applicable to vessel with gas only. If liquid present, results will largely deviate.

You said that "System being depressured is maintained as gaseous throughout entire depressuring process". But hysys depressuring utility sizes blowdown valve for both gas and liquid. What does it mean?

Grote equation only applicable to vessel with gas only. If liquid present, results will largely deviate.

If we are assuming Zero flash, would it be valid?

[JoeWong]

You said that "System being depressured is maintained as gaseous throughout entire depressuring process". But hysys depressuring utility sizes blowdown valve for both gas and liquid. What does it mean?

Grote equation only applicable to vessel with gas only. If liquid present, results will largely deviate.

If we are assuming Zero flash, would it be valid?

I believe there is minimum liquid phase (i.e. 2% or 3% or...) lead to low flashing can still be acceptable. However, this amount has not been established. Probably this is a good assignment to some one interested in this area.

Another typical two phase system has minimum / no impact is Hydrocarbon gas (without liquid phase) with present of water phase only. The gas phase will depressure whilst water stay as water or ice.

[Propacket]

You said that "System being depressured is maintained as gaseous throughout entire depressuring process". But hysys depressuring utility sizes blowdown valve for both gas and liquid. What does it mean?

Grote equation only applicable to vessel with gas only. If liquid present, results will largely deviate.

Can you please provide me a copy of Grote Equation with units system used?

[m_nimgaon]

I have a topside piping volume of 43 m3 which I wish to depressurize from 233 barg to 6.9 barg in 15 min

I have tried to do it in HYSYS dynamic depressurization utility & generated strip charts.

How to estimate Cv ?

What is the difference between Vapour @TPL1 Mass Flow and Vapour Out @TP mass flow ?

Attached Files

•  untitled.JPG   130.88KB   85 downloads

[fallah]

I have tried to do it in HYSYS dynamic depressurization utility & generated strip charts.
How to estimate Cv ?

m_nimgaon,

As far as i know, the Cv is one of the output information of running the HYSYS dynamic depressurization utility.

Fallah

[Propacket]

Joe,

I had a look on your article, "De pressuring Flow - Quick Manual Method". It states,

“Higher the depressuring rate, lower the temperature can be experienced by the depressuring system.”

I completely agree with your statement. But last month I sized a blow down valve of a compressor skid for two different blow down periods. Our client is a renowned gas processing company. They reverted with the following reply:

“We do not understand why the minimum temperatures are different for two time periods. Minimum temperature is influenced by the system volume and the initial starting pressure and temperature. The blow down duration does not affect the minimum fluid temperature”.

Could you provide your expert opinion on this issue?

[fallah]

P.Engr,

Can you submit the initial/final conditions (pressure, temperature, time duration, minimum temperature, depressurization case,...) for two periods which have been submitted to your client? It might be helpful in giving proper input.

Fallah

[Propacket]

Hi Fallah,

Below is the required information. Please note that BDV was existing and client wanted to know the minimum temperatures experienced for two different blow down times, 15 mins and 15sec. The 15 sec time is actual time for which orifice was sized by an engineering services company. I do not know the logic for this short time.

Attached Files

•  Dynamic Depressuring Results.xls   20KB   697 downloads

Edited by P.Engr, 17 April 2012 - 09:13 AM.

[fallah]

P.Eng,

The inventory hasn't been specified in the information. Considering such low volume of depressuring (4 ft3), is the blowdown performed for inventory which is trapped in a pipe segment?

Actually, as per API 521 there is no need to consider depressurizing system for small size equipment. Please clarify this issue and also specify the inventory and that it is inside a vessel or a pipe segment.

Fallah

[Propacket]

Yes the blow down is for a pipe segment. There is no significant liquid inventory in the piping.

[fallah]

P.Engr,

I still stand on my opinion that it don't need to have depressurizing facility.

Anyway, the inventory (fluid type) hasn't been specified yet. Also the orifice size in two cases (15 seconds and 15 minutes periods) couldn't be identical (you mentioned 11 mm for both). Please clarify.

Fallah

Edited by fallah, 19 April 2012 - 12:38 AM.

[Propacket]

Fallah,

It was a type error. The orifice size for 15 mins time is 4 mm.

The fluid is natural gas.

Lets assume whatever you are saying is correct (We should not use a BDV here) and sum up our discussion. I have a simple question: Does the blow down duration affect the minimum fluid temperature?

Edited by P.Engr, 24 April 2012 - 08:39 AM.

[kkala]

I think http://www.cheresou...ow-temperature gives some clarifications.
Upstream of depressurizing device (valve, orifice) temperature corresponds to saturation pressure, as long as there is liquid. Minimum fluid temperature (if there is still liquid) would correspond to flare header pressure and would not depend on depressurization rate
Downstream of the device, temperature is expected to be same as upstream in case of insulated piping. If not, and heat gain from ambient air is considered, downstream temperature may be higher. In this case fast rate would mean lower temperatures instantaneously (little time for heat exchange).
But for a small diameter pipe (not a flare header) these may be theoretical considerations. A simplified view is to consider no heat gain from ambient, so that all pipe gets the temperature of the saturated liquid at the operating pressure. This seems realistic for the case, rather on the conservative side, and complies with Client's opinion.

Note: Header min design temperature (as applied here) neglects heat gain from ambient (conservative).

Edited by kkala, 24 April 2012 - 02:59 PM.

[Propacket]

Thanks kkala

[fallah]

I think http://www.cheresou...ow-temperature gives some clarifications.
Upstream of depressurizing device (valve, orifice) temperature corresponds to saturation pressure, as long as there is liquid. Minimum fluid temperature (if there is still liquid) would correspond to flare header pressure and would not depend on depressurization rate
Downstream of the device, temperature is expected to be same as upstream in case of insulated piping. If not, and heat gain from ambient air is considered, downstream temperature may be higher. In this case fast rate would mean lower temperatures instantaneously (little time for heat exchange).
But for a small diameter pipe (not a flare header) these may be theoretical considerations. A simplified view is to consider no heat gain from ambient, so that all pipe gets the temperature of the saturated liquid at the operating pressure. This seems realistic for the case, rather on the conservative side, and complies with Client's opinion.

Note: Header min design temperature (as applied here) neglects heat gain from ambient (conservative).

P.Eng,

With the above statement seems you did get your response, hence would you please answer to your own question mentioned in 20th post as follow:
...I have a simple question: Does the blow down duration affect the minimum fluid temperature? ...

Fallah

[kkala]

Fallah, clarifications had better come from the writer, if a point looks unclear. Posts No 21 indicates minimum fluid temperature independent of blow down duration in this specific case. Minimum temperature downstream depressurization (and at some distance) could be lower in case of short depressurizing time, if heat gain from ambient air is taken into account. But look into the link referred in post No 21 (http://www.cheresour...w-temperature/ , especially last posts).
Hope present and mentioned posts make stand point more clear, subject to criticism / amelioration as usual.

[ankur2061]

All Design Engineers,

Presently I am studying a report on depressurization of natural gas from well flowlines. The report mentions that the calculated vapor outlet temperature downstream of the RO is -70°C which is below the design temperature of LTCS of -46°C and which would not allow LTCS to be used under such circumstances. The depressurization studies were conducted using HYSYS dynamic depressurization utility.

It is also well know that the HYSYS heat transfer correlations are not accurate enough and
using HYSYS in such a case would invariably give lower temperatures forcing the use of alloy steels such as Duplex Stainless Steel for the pipe downstream of the restriction orifice.

OLGA simulator also has capabilities to perform depressurization studies and it goes one step further in accurately defining the boundary layer between the vapor temperature and the inner wall temperature of the pipe. The report mentions that when they used OLGA for the depressurization studies, a minimum vapor temperature of -70°C resulted in an inner wall temperature of -46°C of the pipe downstream of the orifice due to heat transfer effects of the pipe metal with the outside surroundings. The report concludes that on the basis of this study done in OLGA it has been decided to retain LTCS for all the depressurization lines where the vapor temperature has been calculated to be -70°C or above.

This is for the information of all those who are involved regularly in doing depressurization studies using HYSYS Dynamic Depressurization utility. Remember there is a major cost difference between LTCS and DSS. You will be doing the client a great service if you can help him save the cost of switching from LTCS to DSS based on accurately evaluating what the pipe metal can see as the lowest temperature during a depressurization operation.

Regards,
Ankur.