Pleckner,
I do not have operations background and hence may be little ignorant about operator behaviour. Consider this post as my way of getting first hand information on operator behaviour.
However, as a designer I would argue against using 200 psig upstream pressure on two counts: technical and administrative.
Technical arguments:
1) When I assume upstream system at design pressure, I assume pressure control system in upstream vessel is not working. Even if one does not consider favourable response of instrumentation system, it is highly unlikely that upstream vessel would reach design pressure due to something that happens downstream.
2) It depends on upstream vessel make-up stream. If make-up is much less than outflow, pressure in upstream vessel would actually diminish.
3) Consider make-up is significant. Now gas blow-by occurs with upstream vessel reaching design pressure. Due to increased pressure in upstream vessel, make-up would decrease (you should not consider negative response of make-up control system).
4) It depends on relative volumes of upstream and downstream vessels as well. In one particular case of ours, low make-up stream flowrate and large volume of low pressure system almost eliminated gas blow-by case.
5) Assume, I have sized PSV considering 150 psig in upstream. Now assume control valve fails during operation and upstream vessel somehow reaches design pressure. It still may not be an issue as following margins may help:
- additional flow is taken care of by rated flow through orifice
- Pressure in downstream vessel may exceed design pressure, but since MAWP is generally higher than design pressure no problem occurs
- Combined flow of PSV and downstream pressure control system (even without favourable response) at relieving pressure may avert the situation
6) Generally PSV for gas blow-by case is calculated using bypass valve full open. When gas blow-by occurs, bypass may not be open providing sufficient margin. Here I assume, administrative controls prohibit operators from keeping bypass open in their bid to increase throughput.
Administrative arguments:
During my limited interactions with site people I found them to be serious about PSV operation. If they think something can lead to PSV blowing, I have seen them avoiding it. Also, system of permits to be obtained from technical departments would also help avoid intentional maloperation.
Summary:
I would consider design pressure of upstream vessel (200 psig) for gas blow-by case only I see a chance of upstream vessel reaching design pressure on control valve failure. E.g. upstream vessel level is controlled by a level control valve in its feed.
I would explore such possibilities during HAZOP
Regards,
Sachin