6 replies to this topic

[J_Leo]

Hello,

I estimated the flow rate for control valve failure case is more than doubled compared to the normal through the CV.

I think that once the heat exchanger is selected with a design duty and fixed surface area, it is basically impossible for the heat transferred to double. This means that it is too conservative to assume the relief rate is simply the methanol that can be vaporized due to the extra LP steam supply available (Q=m.ΔH) , regardless of the heat transfer process (Q=U.A.ΔTlm) itself.

The more appropriate way is to estimate Q by estimating U and ΔTlm for the CV fail open case. However, it is very difficult to do so as a process engineer. I am thinking it might be possible if we have HRTI used by Heat Transfer Engineers.
Anyone has a better and yet simple way to estimate the relief rate for this case?

Thank you.

Regards,
Leo

Edited by J_Leo, 11 April 2016 - 09:16 AM.

[Bobby Strain]

You seem to know what your system is. But we don't know. Don't make us guess and give you silly responses.

 

Bobby

[fallah]

J_Leo,

 

Agreed with Bobby; then please upload a simple sketch of the system along with the supporive info...

[Pilesar]

Heat transfer CAN double. The design duty for the exchanger is the minimum required duty for a fully fouled exchanger. The exchanger itself will be larger than required to make sure it meets the minimum requirements. In addition, the heat exchanger will start up completely clean and may can do much more than the design duty if enough steam is available. You will need to re-rate the exchanger for a clean surface (no fouling factor) to calculate the maximum duty possible. One option is to install a restriction orifice in the steam line to physically limit the steam flow if your control valve is too large.

[J_Leo]

Hello Pilesar,

 

Thank you for the inputs. I also think we need to rate the exchanger. Is it possible to do so without the installation of HRTI? I don't have any experience to rate an exchanger. Is this a common way to calculate the relief load for the CV fail open case on the heating media side?

 

Leo

[Pilesar]

The 'common' way is to rate the PSV assuming all the steam that can pass through the control valve condenses. This gives the most conservative value for the PSV and is 'safe'. Sometimes it is not worth the engineering effort to optimize the PSV size unless the calculated PSV is very large. There are ways to reduce the required size of the PSV by adding a restriction orifice to the steam line, reducing the control valve CV, reducing the steam bypass valve CV, or rigorously rating the heat exchanger maximum duty. Rating the exchanger is best done with software and it requires some expertise in the software so I do not recommend your basing a relief valve size on first-time HTRI usage. If the exchanger is a distillation column reboiler, the maximum column pressure may be limited by the temperature approach in the exchanger as the column pressure increases. This limitation often arises for vacuum columns where an increase in pressure may be self-limiting as the heat exchanger reboiler stops boiling at a certain pressure.

[J_Leo]

Pilesar,

 

Thank you very much for the information.

 

Regards,

Leo