Question

b) A process and instrumentation diagram of a flash drum is shown in Figure 05.3. Steam is condensed in a steam coil to vaporize a portion of the liquid feed, and the liquid product is removed by a pump. There are control valves for the steam flow, steam chest (which allow the steam chest to be rapidly evacuated in emergency situations), vapour product, liquid product and feed flow. Vapor Va PT PC FT LT LC Vi Steam Liquid Condensate Figure Q5.3 For a case of reducing vapour flow at downstream to avoid process upset, selecr correct fail-safe position for every control valve (VI,V2,v3,V4,V5) and provide reason (see Table Q.5). Note that, you are required to redraw Table 05 in the answer booklet) (10 marks) Table Q5 Fail-close (F/C) or Fail-open (F/O) Valves Reasons VI V2 V3 V4 V5 (Total: 20 marks)

Answer 1

Valves	Fail-close (F/C)or Fail-open(F/O)	Reasons
V!	F/O ( is air to close valve)	Reason is we want to increase feed temperature(TF), so that it can vaporize. Let increased feed temperature is T1 . So till the time TF<T1 our process fails and as it's F/O valve, it opened the valve so steam will come inside and increased the temperature and for F/O or air to close valve we have to increase the pressure and that will open the valve.
V2	F/O	Initially steam temperature(Ts) > feed(TF) and desired temperature(T1) both.But as Ts<T1our process will fail and valve will open so that condensate steam will come out of the flash drum.
V3	F/C	We want to Reduce vapour flow(V3) at downstream. When V>V3 our process fails and we want to close the valve so that vapour flow can decrease
V4	F/O	As V3  is decreasing means V4  (liquid flow rate coming out at upstream)will increase so as (any liquid flow rate V) V< V4 our process will fail so to increase V again we have to open the valve
V5	F/C	If we increase feed flow VF our process fails and then we want to close the valve.

For better understanding take some values for V₁, T_F etc and then think if want to increase the value we have to close or open if we close it will accumulate inside the flash drum which will increase temperature or flow respectively.