9 replies to this topic

[jawbkk]

We are installing a 4000 gallon pressurised deaerator vessel in our boiler house.
The steam supply is 11.5 barG and saturated. (Assuming 95% dry)
The vessel is to operate at 0.2 barG.
The condensate return is only 30% of the feed water to the vessel.

What should the pressure vessel safety valve pressure be set at, and also where should it be located?

Best regards
John

[fallah]

john,

 

Please upload a simple sketch of the deaerator system also specify design pressure of the deaerator vessel...

[ankur2061]

John,

 

Normally equal to the 'design pressure' of your deaerator as specified in the vessel data sheet.

 

You are operating your deaerator at 0.2 barg and supplying steam from a source which is at 11.5 barg which means you are reducing the pressure across a control valve from 11.5 barg to 0.2 barg plus liquid static head at the point where you are injecting the steam in the deaerator. In case your control valve fails open, the deaerator can see a maximum pressure of 11.5 barg. In such a case the 'design pressure' can be fixed at 11.5 barg if you are yet to decide the design pressure.

 

Regards,

Ankur

[fallah]

john,

 

Deaerator vessels should have the same design pressure as that of the steam supply header to prevent overpressure during pressure control valve failure in full open position. Then the design pressure of the steam header in your case, which is certainly higher than 11.5 barg, would be as deaerator design pressure hence the set pressure of the relevant pressure safety valve... 

[S.AHMAD]

1. The normal practice is Design pressure = operating pressure + 10% (or operating pressure + 25 psig whichever is greater)

2. This means that the design pressure could be 0.2 + 1.7 = 1.9 so we can design your de-aerator at 2 bars. (This is the pressure if I were you).

3. However, you must size the PSV correctly. That means yo need to determine the steam flowrate or the capacity of your PSV based on the maximum fowrate of sream when steam breakthrough due to fully opeing of control valve (e.g. stuck at open position).

3. Alternative, you cou could follow as suggested by Ankur and Fallah. It is a matter of economics which one is  the most economical.

Edited by S.AHMAD, 19 November 2013 - 04:56 AM.

[ankur2061]

1. The normal practice is Design pressure = operating pressure + 10% (or operating pressure + 25 psig whichever is greater)

2. This means that the design pressure could be 0.2 + 1.7 = 1.9 so we can design your de-aerator at 2 bars. (This is the pressure if I were you).

Ahmad,

 

In this case, what you suggest should not be applied. Failure of steam control valve in open position (stuck open) can rapidly increase the pressure in the deaerator far beyond the value of 1.9 bar as suggested by you. If you provide a relief device set at 1.9 barg then the chances that this relief valve will get actuated frequently due to the slightest malfunctioning of the steam control valve are very high.

 

In fact, I would also suggest that the design pressure should be the higher of the maximum steam pressure and the shut-off head of the condensate pump feeding the deaerator.

 

Also, a relief device set at 1.9 barg will be much bigger (higher capacity) than a relief device set at 11.5 barg (lower capacity).

 

Hence, providing a design pressure of 11.5 barg or pump shut-off, whichever is higher makes more logic than providing a design pressure of 1.9 barg for the deaerator.

 

Regards,

Ankur.

[S.AHMAD]

Ankur

1. You have good points for duscussion.

2. The deaerator pressure may mommentarily increase higher than set pressure of 2 barg due to sudden opening of the steam pressure control valve that is normally used to control the deaerator presure (I assume in this case). This can happen if the control valve is failed open (F.O) which I strongly believe it is failed closed (F.C.).  Further more, pressurised deaerator is normally vented to atmosphere with no venting block valve and thus the pressure at 100% steam velve opening the deaerator pressure shall be lower than the steam pressure of 11.5 barg. The deaerator pressure may momentary higher that the PSV set pressure of 2 barg but this will quickly drop to the set pressure provided the PSV is correctly sized for the service. Sudden increase in pressure is unlikely if the valve is stuck open 100% due to mechanical failure of the valve.

3. I agree with you the PSV size at set pressure of 2 barg is bigger than that of 11.5 barg. This is where the economic dictates the decision. The choice is whether thicker deaeretor vessel plus smaller PSV of thinner vessel with bigger PSV. Probably the optimum pressure is betwen 2 barg  and 11.5 barg..

4. Condensate shut-off pump pressure, in my view is irrelevant since the dearetor is vented to atmosphere unless the deaerator piping configuration is not the one that I have in current view. I need to look at the P&ID before making further commments.

Edited by S.AHMAD, 19 November 2013 - 08:22 PM.

[jawbkk]

Thank for your interesting points gentlemen.
The 11.5 bar steam pressure to the steam control valve has a flow rate of 6700 m3/h (mass flow rate to provide the enthalpy to the makeup water)

This flow rate then splits to enter two separate 'heads' with atomizing nozzles for the make up water for maximum oxygen separation.

The vessel is controlled at 0.21 barG to maintain a water temperature of 105 deg C.

The points you have all raised are exactly what I have been thinking in that the pressure in vessel could rise above the design pressure of 2 barG. So I will check the safety valve flow rate for the failed steam control valve condition.

Many thanks again.

Best regards
John

[curious_cat]

Ankur

2. The deaerator pressure may mommentarily increase higher than set pressure of 2 barg due to sudden opening of the steam pressure control valve that is normally used to control the deaerator presure (I assume in this case). This can happen if the control valve is failed open (F.O) which I strongly believe it is failed closed (F.C.).  Further more, pressurised deaerator is normally vented to atmosphere with no venting block valve and thus the pressure at 100% steam velve opening the deaerator pressure shall be lower than the steam pressure of 11.5 barg. The deaerator pressure may momentary higher that the PSV set pressure of 2 barg but this will quickly drop to the set pressure provided the PSV is correctly sized for the service. Sudden increase in pressure is unlikely if the valve is stuck open 100% due to mechanical failure of the valve.

3. I agree with you the PSV size at set pressure of 2 barg is bigger than that of 11.5 barg. This is where the economic dictates the decision. The choice is whether thicker deaeretor vessel plus smaller PSV of thinner vessel with bigger PSV. Probably the optimum pressure is betwen 2 barg  and 11.5 barg..

 

 

@Ahmad:

 

Very interesting points about the tradeoff. Two thoughts:

 

(1) Even if valve is of FC design that only  deals with the failure mode of instrument air supply loss, correct? There can be other credible scenarios where valve does stick open due to other malfunctions. Not sure what are typical operational failure modes of a valve in this duty. 

 

(2) Out of curiosity, can anyone throw ball park values / estimates of the costs associated with a PSV for this duty at both 2 barg & 11.5 barg versus the differential vessel cost of a 4000 gallon pressure vessel designed to the higher & lower pressure specs.? Not implying cost is the factor that decides but I'm just curious.  

 

Order of magnitude, what are these figures. 

[S.AHMAD]

curious_cat

1. Yes it is possible for thr valve to be fully open during operations. In HAZOPS normally we just say that control valve is "stuck open 100%" without studying more details the real cause of stuck open position.

2. I have no idea about the cost comparasion. Appreciate if anyone can share his/her experience.

 

John,

1. You need to compute the maximum break through steam flowrate across the valve when it is fully open. This nomally occur at the steam critical pressure which is normally about 50% of the upstream pressure. Thus in this case the critical pressure is about 5 barg. That is a pressure drop of 6.5 bar. Using the control valve equation and the valve C_V (size), you can compute the maximum flowrate.

2. Two scenarios for the PSV namely fire case and steam break trough case. However if the deaerator is located higher than 25 ft above ground then fire is not a credible case.

Edited by S.AHMAD, 24 November 2013 - 06:07 PM.