De-inventory safety standby systems may sit for years before use. In fact, unless they are intended for unit shutdowns as well, the objective is never to activate them. However, when called upon, the systems should handle extreme circumstances reliably and with minimum operator intervention.
Liquid de-inventory systems fall into four categories: pressurized; displacement; gravity drain; and pump-out. In pressurized systems, a high-pressure reservoir of gas forces liquid from the system into a reservoir. In displacement systems, a liquid displaces the process flow and forces it to a destination. In gravity drain systems, liquid can empty into a reservoir at a lower elevation. In pump-out systems, a pump moves the liquid to a safe reservoir outside the unit. Pressurized and displacement systems are relatively rare. Plants commonly opt for gravity drain and pump-out systems to transfer liquid inventory.
Gravity drain systems work reliably as long as the process elevation, drain reservoir elevation and connecting piping are adequate. Often, though, e.g., when a vessel needing emptying is at or near grade, there’s no reasonable way to make the elevations work.
For pump-out systems, no widely accepted design rules exist. Different philosophies and regulatory regimes can lead to disparate pump-out system choices. So, let’s briefly look at some areas needing decisions.
Determining the net positive suction head available (NPSHA) to use for a pump-out system requires answers to questions about three major factors:
1. Will compositions be changing dramatically from normal operation? One example would be all the liquid from the trays in a tower dropping into the tower bottoms and needing pumping out. On large towers with steep composition profiles, this can result in vaporizing mixtures in the tower boot. The head required (NPSHR) to prevent vaporization in the pump suction may be less than that for the usual assumption of bubble-point liquid.
2. Will system pressure be at normal conditions? Do the pump-out contingencies include loss-of-containment? In this case, the pump must operate while the system is depressurizing. Again, the pumped liquid may contain vapor.
3. Must the pump drain the system to very low liquid levels to remove as much inventory as possible? Here, the NPSHA should reflect the low liquid level.
There are no easy solutions for pumps that will have vapor in the feed during pump-out. The best course of action is to reduce NPSHR and use a slower-speed pump. A 1,800-rpm pump can tolerate more abuse for the same conditions than a 3,600-rpm pump. Don’t rule out even lower speeds. In one case, I specified 900 rpm for a pump-out system.
Pump-outs often occur when operators are fully engaged in dealing with other issues. So, a base assumption is that once the pump-out system is turned on, the operator doesn’t have to think about it again.
Low NPSHR pumps often suffer problems with suction recirculation. They should have a recirculation line to alleviate this. To make the system as reliable as possible, use an orifice in the recirculation line rather than rely on an active control valve for low-flow protection. This mandates increasing the pump capacity.
The flow recirculation loop shields against suction recirculation and gives some protection against temperature rise during blocked flow.
If the downstream flow is blocked, the work going into the pump will pass into the recirculating liquid. That liquid may recirculate inside the pump or in an external loop. In either case, unless there’s heat removal in the loop, the temperature of the liquid will increase. An external loop has larger inventory, so temperature will rise more slowly even without external cooling.
Unless the recirculation loop has some method of heat removal, the recirculation will slow — but not eliminate — the effect of fluid heating. If the loop includes a cooler or the recirculation goes upstream of the pump to a location where it can cool, then the recirculation loop will help lower pump heating.
Adding recirculation to the pump suction increases pump reliability. The recirculation loop helps reduce problems from suction inlet recirculation and, at a minimum, cuts the impact of fluid heating at zero net flow.
Opting for a slow-speed pump and a robust design with a recirculation loop generally offers significant advantages — including, importantly, minimizing the need for attention by operators when they might be very busy.