Question from March's Chemical Processing
A decanter serves a distillation column used to separate water from solvents. Recovered solvents are then burned in a thermal oxidizer (TOX). The decanter separates the heavy organics, such as carbon tetrachloride, from water. The tank diameter is 48 in. and has a volume of 1,000 gallons. During start-up, it is noticed that the minute the pump begins to fill the tank the high level switch, 90° from the tank inlet, trips the pump off (Figure 1). What can be done quickly to eliminate this problem and keep the overflow safeguard?
Figure 1. Splashing causes high level switch to trip — killing pump prematurely.
Change the pump discharge nozzle
Why not a simple 45°, or 90° ell. Also, a baffle, wrapped around the level sensor, on the pump inlet side. And, there’s always an extension of the inlet down into the fluid, as the ultimate choice. I could go on with siphons, for creating low flow rates.
Tom Murphy, CEO
Puritrol, Inc., Centerville, Mass.
Install an elbow
With installation an elbow at the pump outlet to the decanter we can prevent pump trip.
Jahanian, project engineer
Industrial Projects Management of Iran, Tehran, Iran
Consider three potential solutions
Since we know so little about the cause of the problem, there are a number of solutions, but they reduce to a mechanical solution, an instrumentation solution or a control solution. If this problem exists only during the initial starting of the pump, the level switch could be bypassed for a specific period of time during pump start only. This would be the quickest but not the best solution. However, if the problem persists even after the system reaches steady state then this solution won’t work.
From an instrument perspective, choose a different switch technology. The selected technology should not be affected by the splashing in the tank. An internal stilling well and either a float switch or a radar-type level transmitter inside of the well would be ideal. The well would need to be constructed and oriented so that the openings in the well would be 180°away from the inlet piping. This would have an added advantage of (depending on instrument selection) being able to detect the interface between the two liquids.
Alternatively, a deflector could be added to the end of the sensor pipe to prevent the wholesale flooding of the switch that is occurring now. Since we know almost nothing about the switch except that it appears to be mounted at the outside end of a piping nozzle, this solution could be quick and effective.
The mechanical solution is the one of choice. Add a diffuser to the end of the inlet piping to minimize splashing and turbulence. This should not only eliminate the unwanted trips from the level switch but also greatly improve the separation in the decanter. The decanter should be fitted with several baffles to aid in separation, if they don’t already exist. This will take some time, unfortunately, as it would need to be designed, fabricated and installed. Also the flow rate from the pump into the decanter should be reduced, if possible, to reduce turbulence. An unconventional and quick solution would be to add a flow straightener consisting of numerous small tubes near the end of the inlet pipe at the vessel to develop laminar flow into the vessel and eliminate splashing.
Patrick Richards, senior instrumentation designer
Irving Oil Ltd., Saint John, N.B.
Employ a relay
Here is a process control solution that involves no hardware modifications to the decanter system. This is a low cost and easy solution. The problem appears to be that when the pump starts a sudden inrush of fluid causes a surge in liquid height which then trips the high level switch. Most likely the momentary surge in liquid height dissipates after a few seconds of the pump running.
The solution to avoid the high level switch from tripping the pump is to install a delay in the high level trip on the pump start circuit. This can be accomplished by installing a hardware or software relay switch that needs to be latched for a set time period before the pump start circuit is opened. If the high level switch interlock is in a PLC or DCS control system, try starting off with a 10-sec. delay and then start increasing the delay time until the pump stays on without tripping out. Adding a delay time of seconds to the pump interlock should not increase the risk of the tank overflowing when the level in the tank reaches the high level switch since there is always a buffer distance between the top of the tank and the high level switch.
Mike Dobrowolsky, site utilities leader
GE Plastics, Selkirk, N.Y.
Put in a still well
You didn’t say what type of level switch was being used to shut off the pump. One thing that could be done if it’s a ball float level switch or an ultrasonic probe is to put it still well. This will stop a splashing action that is affecting the switch when the pump is turned on.
Pete Bird, warehouse supervisor
Pioneer Americas LLC, St. Gabriel, La.
Eventually, do a redesign
The problem description suggests that the liquid level in the tank is too high when the liquid isn’t being decanted. The design has flaws but it isn’t the place to criticize it, we need to solve a problem. I would consider the following:
- Lower the liquid level when the feed pump starts so that there’s no splashing of the liquid to activate the high level shut off.
- Install a 90° elbow to introduce the liquid at a good distance below the high level sensor. This will prevent any splashing contact.
For the long term, I would modify the design. Liquids with the density difference of water and carbon tetrachloride and similar liquids can be separated by using a natural gravity decanter with a proper design. They work extremely well and do not require such a high volume tank. Due to the high density difference, the separation is usually quick and clean. Such decanters work fine without any problems, automatically, and continuously. No level controllers are needed; I have designed many decanters and so can state with experience that this one needs to be redesigned.