SNORGY
Mechanical
- Sep 14, 2005
- 2,510
I feel somewhat incompetent here, but hopefully someone might have run into this before.
There is an indirect fired line heater heating sour natural gas (single phase, largely methane, MW = 17.72, 7930 kPag @ 25 C inlet) between two sections of a pipeline. The bath is 50%-50% EG-Water controlled to 88 C. The inlet and outlet piping and the line heater coil between the upstream and downstream ESD valves have been called up to ASME B31.3 2006 using ASTM A333-6 / A350-LF2 Class 1, 600# ANSI. It is unclear whether or not a plant ESD results in shutting off the gas supply to the heater, although locally there are expansion tank level and bath temperature cut-outs.
Oddly enough, the original design basis from the line heater vendor was 15 MMSCFD of gas - in which case the existing PSV was adequately sized for blocked flow anyway. The problem came when the pipeline throughput was increased to 25 MMSCFD and the line heater was still big enough to handle it.
The rated duty of the line heater is 4.25 MMBTUH, but the process is using only one coil (the reheat coil) that consists of 360 feet of 6" SCH 120 pipe and is approximately 52% of the total coil surface area for all coils in the unit. The temperature rise of the gas through the heater is approximately 41 C. In any case, the field operations personnel are currently able to flow 25 MMSCFD of gas and achieve a 41 C temperature rise. On that basis, I arrive at an actual heat input rate of about 928.5 kW (3.17 MMBTUH) using Cp = 0.655 BTU/(lb-F).
There is an existing PSV on the heater coil outlet. It is stamped for 10,875 CFM, which I assume means SCFM of air. It is set at 9930 kPag, and at a calculated (HYSYS) SG of 0.78, I arrive at a capacity 12315 SCFM of gas.
The Regulatory Body's A.I. is in receipt of drawings that do not show this PSV and is asking how the system is protected from overpressure for a case in which the line heater is blocked in (i.e., plant ESD) but continues to add heat to the system. In my mind, the answer appears to be "...It isn't...", and the corresponding rise in pressure for the stagnant gas to the maximum bath temperature of 88C would be (but for the PSV) to 10,480 kPag (HYSYS). The coincident flange rating is 9490 kPag. If the pressure rise is limited by PSV set point, the (P,T) condition will be (9930 kPag, 74 C) while the coincident flange rating will be 9690 kPag.
With the above said, my questions are:
(1) For the blocked-in (both upstream and downstream) case, can Operations make use of Clauses 322.6.3(b)(1) and 302.2.4(f)(2) as a self-limiting event (relieving case) in order to accept the existing PSV setting at 9930 kPag with coincident flange rating at 9690 kPag?
(2) For a case where only the downstream (outlet) from the heater is shut in and the line pack inventory from the upstream pipeline is available to continue to flow into the line heater, is it reasonable to assume that the maximum flow achievable will be exactly the capacity of the relief valve, such that the maximum pressure that can be developed in the gas in the line heater and connected piping would be coincident with that determined by the equation of state corresponding to the maximum bath temperature of 88 C? This would require coincident failure of the ESD valve on the upstream pipeline (i.e., failure in the CSA Z662 pressure limiting system) and the computed (P,T) rise (HYSYS) would be to 10480 kPag @ 88 C with a coincident flange rating of 9490 kPag. Again, the same question applies as for #(1).
(3) Irrespective of the above, should a relief valve sized for a blocked flow case be mandatory? The problem that I have is that, under the conditions described above, the only way that the enthalpy gained via the heat input at the line heater is by the enthalpy lost with the gas relieved via the PSV, and my thoughts are that for a single phase gas if you have enough duty available to heat the gas at a flow rate in excess of the relief valve capacity, you cannot shed enough enthalpy through the relief valve to prevent a continued pressure and temperature rise to the limits imposed by the bath temperature.
Oddly enough, the line heater vendor's original design basis was to heat 15 MMSCFD of gas by 41 C, in which case the original PSV was adequately sized for the blocked flow case anyway. The problem came when the pipeline throughput was increased to 25 MMSCFD and the line heater was big enough to handle it.
Regards,
SNORGY.
There is an indirect fired line heater heating sour natural gas (single phase, largely methane, MW = 17.72, 7930 kPag @ 25 C inlet) between two sections of a pipeline. The bath is 50%-50% EG-Water controlled to 88 C. The inlet and outlet piping and the line heater coil between the upstream and downstream ESD valves have been called up to ASME B31.3 2006 using ASTM A333-6 / A350-LF2 Class 1, 600# ANSI. It is unclear whether or not a plant ESD results in shutting off the gas supply to the heater, although locally there are expansion tank level and bath temperature cut-outs.
Oddly enough, the original design basis from the line heater vendor was 15 MMSCFD of gas - in which case the existing PSV was adequately sized for blocked flow anyway. The problem came when the pipeline throughput was increased to 25 MMSCFD and the line heater was still big enough to handle it.
The rated duty of the line heater is 4.25 MMBTUH, but the process is using only one coil (the reheat coil) that consists of 360 feet of 6" SCH 120 pipe and is approximately 52% of the total coil surface area for all coils in the unit. The temperature rise of the gas through the heater is approximately 41 C. In any case, the field operations personnel are currently able to flow 25 MMSCFD of gas and achieve a 41 C temperature rise. On that basis, I arrive at an actual heat input rate of about 928.5 kW (3.17 MMBTUH) using Cp = 0.655 BTU/(lb-F).
There is an existing PSV on the heater coil outlet. It is stamped for 10,875 CFM, which I assume means SCFM of air. It is set at 9930 kPag, and at a calculated (HYSYS) SG of 0.78, I arrive at a capacity 12315 SCFM of gas.
The Regulatory Body's A.I. is in receipt of drawings that do not show this PSV and is asking how the system is protected from overpressure for a case in which the line heater is blocked in (i.e., plant ESD) but continues to add heat to the system. In my mind, the answer appears to be "...It isn't...", and the corresponding rise in pressure for the stagnant gas to the maximum bath temperature of 88C would be (but for the PSV) to 10,480 kPag (HYSYS). The coincident flange rating is 9490 kPag. If the pressure rise is limited by PSV set point, the (P,T) condition will be (9930 kPag, 74 C) while the coincident flange rating will be 9690 kPag.
With the above said, my questions are:
(1) For the blocked-in (both upstream and downstream) case, can Operations make use of Clauses 322.6.3(b)(1) and 302.2.4(f)(2) as a self-limiting event (relieving case) in order to accept the existing PSV setting at 9930 kPag with coincident flange rating at 9690 kPag?
(2) For a case where only the downstream (outlet) from the heater is shut in and the line pack inventory from the upstream pipeline is available to continue to flow into the line heater, is it reasonable to assume that the maximum flow achievable will be exactly the capacity of the relief valve, such that the maximum pressure that can be developed in the gas in the line heater and connected piping would be coincident with that determined by the equation of state corresponding to the maximum bath temperature of 88 C? This would require coincident failure of the ESD valve on the upstream pipeline (i.e., failure in the CSA Z662 pressure limiting system) and the computed (P,T) rise (HYSYS) would be to 10480 kPag @ 88 C with a coincident flange rating of 9490 kPag. Again, the same question applies as for #(1).
(3) Irrespective of the above, should a relief valve sized for a blocked flow case be mandatory? The problem that I have is that, under the conditions described above, the only way that the enthalpy gained via the heat input at the line heater is by the enthalpy lost with the gas relieved via the PSV, and my thoughts are that for a single phase gas if you have enough duty available to heat the gas at a flow rate in excess of the relief valve capacity, you cannot shed enough enthalpy through the relief valve to prevent a continued pressure and temperature rise to the limits imposed by the bath temperature.
Oddly enough, the line heater vendor's original design basis was to heat 15 MMSCFD of gas by 41 C, in which case the original PSV was adequately sized for the blocked flow case anyway. The problem came when the pipeline throughput was increased to 25 MMSCFD and the line heater was big enough to handle it.
Regards,
SNORGY.
