Direct acting valves
Choose the right valve size needed. Choosing the right size is critical to the longevity of the pressure reducing valve. Over-sized
valves will lead to noise, valve failure and wire draw during low flow conditions. If more than one size of the control valve meets your required
capacity, choose the valve that is less expensive and allows you to use smaller diameter pipes.
Warnings:
Direct acting valves are the most economical type of pressure reducing valves; but they are less accurate than the piston
operated and externally piloted valves.
The cost consists of the energy needed to cool it and re-inject it. It should be fully recovered in the fractionator.I have never heard of anyone using a Plan 32 gas oil flush in the vacuum bottoms service. If they did, this would also cause gas oil to be lost to coke. But, since these pumps are under vacuum, they typically have Plan 54 or some other variation without injected flush.But, for coker charge service, our experts indicate that 10 to 20% of the gas oil will crack in the heater and be converted to coke and light products. The net loss of even 10% of this gas oil would cost at least US$500,000 per unit per year. I base this on the following: Two double ended pumps flushing 3.5 gpm per seal for 360 days per year at a gas oil value of $30 per barrel with 10% of the gas oil cracked into coke.If the seal selected ends up being a Plan 32/52 as proposed, I would prefer a standard tandem arrangement with the primary seal having a rotating bellows. I have always preferred the rotating bellows in "dirty" services such as this. We have had some problems with the rotating bellows in this service because of corrosion/erosion of the shell. In two of our cokers, we use rotating bellows and in the other one, we use stationary bellows. With our planned upgrade to Plan 54, we are going to be proposing a face-to-face stationary bellows with the product on the ID of the primary seal and the barrier fluid on the OD of both bellows. Another refinery owned by our same parent company uses this arrangement in their coker charge pumps with great success.
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2011-09-23
