I'm working on a gas mixing system where I have to inject between 1 to 100 Nm3/hr of air into a natural gas stream in order to stabilize the Wobbe Index of the gas. The injection rate of air is dependent on the flow rate and calorific value of natural gas. The flow rates of the two gas streams are measured by flow meters with temperature and pressure compensation and the flow rate of air is controlled by a 1" V30 control valve.
Due to the high turndown requirement, the control valve has to operate with a low differential pressure to increase controllability at low flow, but the problem is, at high flow, the droop from the regulator will cause the differential across the flow control valve to diminish. Using a pilot operated regulator such as the Type 99 might help, but the 2" might be oversized to give good performance and lockup response at low flows.
One solution I'm considering is to use the Fisher 630 with a remote droop compensator. The setup I envision is to supply an additional pneumatic loading pressure to the diaphragm via the 1/4" NPT vent port and the loading pressure controlled by an I/P converter. The regulator would be set to provide a relatively low discharge pressure say 1.15 barg to provide good controllability at low flow. As flow increases, the additional loading pressure also progressively increases to compensate the droop and also increases the discharge pressure at the high flow range to increase capacity of the flow control valve.
My question is, if the Fisher 630 is equipped with a steel body, will the spring case be able to withstand 1 barg of pneumatic pressure?
Has anybody tried this setup that can share some experience?
In reply to Eric Tam:
Eric,
Yes, the Type 630 spring case can be pressure loaded in the way that you are requesting. With a spring setpoint of 1.15 bar you would be using the Type 630 with a low-pressure actuator, which is limited to an outlet casing pressure of 4.6 bar. Your combination of spring setting + pressure load should not exceed 4.6 bar. You will need to request the special option of a "pressure load seal washer" from your sales office when purchasing. This washer will be installed around the adjusting screw, and will be clamped down by the lock nut. See the picture below. FYI: A Type 627 is a similar regulator option for you that can also be ordered with a seal washer as well.
I hope this is helpful information to help solve your selection needs.
Thanks,
Jason
{seal washer, lock nut, and adjusting screw}