'CASCADED' PILOT REGULATORS HELP REDUCE LPG LOSS IN HOT WEATHER

Fina Oil & Chemical Co. and Fisher Controls International used engineering resourcefulness to overcome heat-induced product loss from LPG storage bullets at Fina's Port Arthur, Tex., refinery.

The installation of pilot-type regulators, in "cascaded," or series, formation, reduced product venting through relief valves. Fina employees Mike E. Youngblood, instrument and electrical supervisor, and Jerry D. Woods, reliability engineering advisor, described the system in an unpublished report.

PROBLEM

Fina had installed Fisher's Easy Joe 399A-6365, a pilot-operated, back-pressure-type regulator, on its LPG storage facility in 1991 as part of a fuel products modernization project.

The regulators helped control the accumulation of noncondensible vapors, which collect in the storage bullets above the LPG. But summer heat extremes and surges in the tanks and lines made it possible for the operating pressure to increase so that the safety relief valve was activated before the pilot regulator was able to stabilize the pressure.

This meant that it was possible that the safety relief valves would open, releasing raw hydrocarbons to the atmosphere before the noncondensible vent relief valves fully opened to the flare.

Excessive LPG was lost to the flare because of the buildup of upstream pressure needed to fully open the vent relief valve. The buildup, or operating range, required to produce full flow was about 22 psig.

Therefore, the initial pressure setting for opening the noncondensible vent relief valve had to be set at 210 psig to ensure that the regulator would be fully opened at 232 psig. Higher pressures were avoided because the bullets' primary safety relief valve pressure was set at 250 psig.

SOLUTION

Reducing the operating pressure range of the pilot system by half or more would enable Fina to increase the noncondensible vent valves' initial opening pressure. This increase would, in turn, reduce the potential loss of LPG to the flare or to atmosphere.

Fina contacted Scallon Controls, Beaumont, Tex., Fisher's local representative. Scallon recommended installing two pilots in a cascaded arrangement. (In a cascaded system, a second pilot controls the main pilot.) This arrangement was designed to decrease the excess vapor-pressure buildup that triggers a stroke of the entire relief valve.

In the summer of 1993, Fina placed Fisher's small 6365C-1 pilot regulator in series with the existing pilot and 399A pressure-relief valve to create the cascade effect and reduce the activation range (Fig. 1).

This approach reduced the total operating range of the noncondensible vent-relief valve to 10 psig from 22 psig. It also allowed Fina to increase the initial pressure setting for opening the valve to 225 psig from 210 psig.

The net effect of this change, say Youngblood and Woods, is that the venting of noncondensibles may be completed with reduced loss of LPG, without reaching a pressure great enough to trigger the opening of the bullets' safety relief valves.

The 6365C-1, available in aluminum or stainless steel, features a plated steel control spring, engineering resin vents, and a heat-treated stainless steel diaphragm plate. The diaphragm limiter is aluminum, brass, or stainless steel, while the plug-stem assembly and body plug 0-rings are nitrile. Gaskets are composition, except for the connector cap gaskets, which are fluoroelastomer.

OPERATING PRINCIPLE

As long as the inlet pressure is less than the set pressure, the 399A pilot remains closed. Inlet pressure bleeds through the fixed restriction in the standard pilot, providing loading pressure to the spring case of the cascade pilot, thus helping the cascade pilot remain closed.

Inlet pressure bleeds through the fixed restriction in the cascade pilot, providing loading pressure to the 399A, which keeps the main valve plug shut tightly. When the upstream pressure reaches the set pressure, the standard pilot begins to open. The standard pilot bleeds loading pressure out of the pilot exhaust faster than it can be replaced through the fixed restriction.

As the loading pressure is reduced, the cascade pilot opens with minimal buildup. The cascade pilot bleeds the loading pressure off of the 399A faster than it can be replaced through the fixed restriction.

As the loading pressure decreases, the inlet pressure opens the 399A main valve. As the inlet pressure decreases to set pressure, the standard pilot closes.

Loading pressure again builds up to close the cascade pilot. The cascade pilot then will provide loading pressure to shut off the 399A main valve.

The set point adjustments are made with the standard pilot because the cascade pilot is nonadjustable.

BENEFITS

Fina is conserving LPG by reducing product loss, which puts dollars on the bottom line. And the frequency and duration of LPG venting, either to the flare or to atmosphere, is reduced. This reduction helps Fina conform to increasingly strict air standards, both state and federal.