7 replies to this topic

[J_Leo]

Hi There,

 

First of all, happy new year to you!

 

We have a PSV located at the inlet of a fired hot oil heater. The intention is to protect the heater tubes for a blocked outlet case.

 

It is pretty straight forward for the PSV sizing if I consider only the thermal relief.

 

The question is if I need to consider the vapor relief from vaporization caused by further firing. Right now I routed the PSV outlet to an expansion drum, which is not designed to handle temperature higher than 600°F. This temperature can be easily exceeded if vaporized hot oil comes in. Because the nature of hot oil, it is not favorible to rout it directly to flare.

 

If I need to consider the vapor relief case, how to calculate the relief load and relief temperature? Right now, I have only the heater duty and I don't have information on either the heating surface area or inventory. The heater will be by the vender. Further more, hysys cannot do flash calculation under relief pressure (around 420psig) to give vapor properties. Therefore, I cannot get laten heat from Hysys. Also the hot oil properties beyond its max operational conditions are not available from the vendor's website.

 

Would you please give some inputs on how to size the PSV?

 

Wish you a prosperous 2014!

 

Regards,

Leo

[Jiten_process]

Hi J_Leo

 

First question is, In what scenario you are anticipating vaporization of hot oil. which grade/type of hot oil u r using. has it been properly evaluated that the vaporization of hot oil is going to take place in any scenario, if yes, which is the scenario. 

 

It is very rare to consider oil vaporization for PSV relief. How excess firing will vaporize the hot oil. are you aware the flashpoint/boiling point of hot oil ? 

Edited by Jiten_process, 02 January 2014 - 03:52 AM.

[S.AHMAD]

Hi Leo

1. Proper design for a furnace/heater to have the fuel supply cut-off when no flow scenarios. Do you have this emergency system?. Under scenario outlet heater blockage, result with no flow and hence fuel supply is cut off. This means no firing continue and hence no vaporization.

2. Therefore, the PSV shall be designed based on pump shut-off pressure and liquid phase.

3. The concern of no flow in a heater is more of tube overheating that leads to tube rupture. That is the reason for the trip system.

Edited by S.AHMAD, 02 January 2014 - 05:05 AM.

[christopher.choa]

Why not locate the PSV downstream of the heater? Block discharge scenario PSVs are often in the downstream of the protected equipment.

[fallah]

Why not locate the PSV downstream of the heater? Block discharge scenario PSVs are often in the downstream of the protected equipment.

 

Hi,

 

Because of possibility of internal diameter reduction or plugging of tubes due to coking

[J_Leo]

S.AHMAD,

 

You got very good points. We do have the trip of fuel gas on low low hot oil flow to the heater. Flow meter is located before the inlet distribution header to the heater. Also, high hight hot oil temperature trips the fuel gas too. The temperature transmitter is located after the header at the heater outlet.

 

Only one thing I am not sure: can we take the trips as substitutes for the PSV for the abnormal heating case? Based on my understanding, usually we cannot take credit of instrument control unless it is a HIPS.

 

The abnormal heating of heater tubes shall include both the blocked outlet case and the fail open of the control valve on the fuel gas supply line. The good thing is the control valve is included in the heater vendor's scope. I plan to put a note on the PSV datasheet to say "The fuel gas control valve  fail open case to be evaluated during EPC". For the abnormal heating case caused by blocked outlet, we don't have to consider it only if we can take the credit of the trips.

 

Thanks,

Leo

Edited by J_Leo, 03 January 2014 - 01:13 PM.

[J_Leo]

Why not locate the PSV downstream of the heater? Block discharge scenario PSVs are often in the downstream of the protected equipment.

 
Hi,
 
Because of possibility of internal diameter reduction or plugging of tubes due to coking

Fallah,
You got very good point too. Another advantage is the relief temperature is lower when it is located at inlet. It is my opinion

Edited by J_Leo, 03 January 2014 - 01:26 PM.

[S.AHMAD]

Leo,

I could not give you any further comments without looking at the P&ID. However my view based on my imagination of the system that I have experienced.

1. With heater discharge valve closed, there are two possible causes of coil/tube overpressure namely pump shut-off pressure and thermal expansion. If the coil design pressure is the same as the maximum possible pump shut-off pressure then the design is good enough. Only to consider thermal expansion case due to blocked in scenario.

2. To eliminate valve closing inadvertently , install the valve such that the valve stem is horizontal and CSO the valve. The best way is not to install any valve downstream of the heater.

Edited by S.AHMAD, 03 January 2014 - 09:34 PM.