10 replies to this topic

[Dmitry]

Hello Dear member,

 

On our Gas plant we have LPG recovery unit with two boilers for stabilizer and debutanizer column. We use heat medium (oil) to keep required temperature in reboilers. To control temperature on outlet heat medium pipe downstream of reboilers we have Temperature control valves.

For me it's interesting why TV's installed downstream of heated equipment because on the same plant we have gas /heat medium exchanger to preheat natural gas if required. And here the TCV is installed on inlet heat medium piping upstream of heat exchanger.

 

If you could please give me some information regarding place of installation TCV. May be it depends from temperature in process or from what?

See attached drawing for reference

 

Regards,

Dmitry

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[juche]

Hello Dmitry,

 

Usually we are having a closed loop for Hot Oil and it is provided with a surge drum, a suction drum, a fire heater and we have multiple users and then Hot Oil is again coming back to surge drum. 

 

The hot oil temperature at the inlet to Exchange is high where as at the outlet it will be lower. So i can analyse both the cases and see if can save on the metallurgy selection for the TCV then i will install TCV at the O/l of the exchange.

 

But if i am installing a TCV at the O/L of the exchange and tube rapture takes place and process side pressure is higher than the Hot Oil Side pressure then i need to install PSV on the Hot Oil Side size it for Tube Rupture Case for all the users individually.

 

Which will be a costly affair so it will be better to upgrade the TVC metallurgy and install it at upstream and keep the Hot Oil O/L valve as Lock Open and install a single PSV at Surge Drum which will be sized such that it can take the highest load of tube Rupture case and fire case also.

 

Thanks

[Dmitry]

Hello Dmitry,

 

Usually we are having a closed loop for Hot Oil and it is provided with a surge drum, a suction drum, a fire heater and we have multiple users and then Hot Oil is again coming back to surge drum. 

 

The hot oil temperature at the inlet to Exchange is high where as at the outlet it will be lower. So i can analyse both the cases and see if can save on the metallurgy selection for the TCV then i will install TCV at the O/l of the exchange.

 

But if i am installing a TCV at the O/L of the exchange and tube rapture takes place and process side pressure is higher than the Hot Oil Side pressure then i need to install PSV on the Hot Oil Side size it for Tube Rupture Case for all the users individually.

 

Which will be a costly affair so it will be better to upgrade the TVC metallurgy and install it at upstream and keep the Hot Oil O/L valve as Lock Open and install a single PSV at Surge Drum which will be sized such that it can take the highest load of tube Rupture case and fire case also.

 

Thanks

Hello and thanks,

 

So do you mean that if shell side design pressure less than tube side we should install TCV upstream?

 

Dmitry

[shan]

Usually, TCV is installed downstream of heat exchanger to avoid vapor flash out across the valve.

[fallah]

 

But if i am installing a TCV at the O/L of the exchange and tube rapture takes place and process side pressure is higher than the Hot Oil Side pressure then i need to install PSV on the Hot Oil Side size it for Tube Rupture Case for all the users individually.

 

Which will be a costly affair so it will be better to upgrade the TVC metallurgy and install it at upstream and keep the Hot Oil O/L valve as Lock Open and install a single PSV at Surge Drum which will be sized such that it can take the highest load of tube Rupture case and fire case also.

 

 

But if the hot oil side would be the shell side with lower design pressure and tube rupture is a credible scenario, anyway a PSV shall be installed on the shells of the all exchangers which will have to be sized for highest load among those of all credible scenarios...
 

[ahmadikh]

 Sometimes downstream is preferred to avoid a thermal shock to the exchanger, in the case of control valve malfunction closure... Also, as stated above the flashing might be an issue at higher temperature

[Dmitry]

Thanks for reply,

 

As I understand from comment abovy installation of TCV downstream heat exchanger is preferred since:

-  to avoid vapor flash out across the valve, at high heat medium temperature

- in caseTCV upstream, when temp is low valve needs open and heat exchanger up to temp which takes a while until stable, then overshoots temp setpoint, valve closes

- it will give smoother control downstream because heat meadium fluid in exchanger if TCV is downstream will be at temperature of heater (minus piping loss)

 

Any another causes?

 

Dmitry

[Zauberberg]

I would also add the following: in your first drawing, hot oil flows through the tube side, whereas in the other drawing hot oil is on the shell side. Shell side is usually designed for low(er) pressure.

 

If the shell side of the exchanger is designed for less pressure than the hot oil pump shut-in pressure, one would need rather large pressure relief device on the exchanger. For tube-side, designing for high pressure is normally not an issue, but for shell-side high pressure design costs a lot. It may be the case that the designer had chosen a TCV upstream of the exchanger to avoid blocked outlet scenario resulting in large PRD.

[juche]

 

 

But if i am installing a TCV at the O/L of the exchange and tube rapture takes place and process side pressure is higher than the Hot Oil Side pressure then i need to install PSV on the Hot Oil Side size it for Tube Rupture Case for all the users individually.

 

Which will be a costly affair so it will be better to upgrade the TVC metallurgy and install it at upstream and keep the Hot Oil O/L valve as Lock Open and install a single PSV at Surge Drum which will be sized such that it can take the highest load of tube Rupture case and fire case also.

 

 

But if the hot oil side would be the shell side with lower design pressure and tube rupture is a credible scenario, anyway a PSV shall be installed on the shells of the all exchangers which will have to be sized for highest load among those of all credible scenarios...
 

 

 

Fallah,

 

I feel we can avoid providing PSV on each exchange if TCV is provided upstream of the exchange in hot oil line and downstream valve is locked open and you have design the shell for 10/13 rule and downstream piping till the isolation valve. PSV will be provided on the surge drum which will be able to take the load of governing case and while deciding governing case you have to consider all the exchanges/users of hot oil into consideration (for tube rupture and other applicable case).

 

thanks

 

thanks 

[fallah]

 

I feel we can avoid providing PSV on each exchange if TCV is provided upstream of the exchange in hot oil line and downstream valve is locked open and you have design the shell for 10/13 rule and downstream piping till the isolation valve.

 

 

juche,

 

Anyway you should consider a PSV on the shell side of each exchanger for code compliance...
 

[Dmitry]

Thansk all for replies!

Regards,

Dmitry