8 sludge drainage pumps

centrifugal pump minimum flow control

What would be the criteria to select the use of an automatic flow control valve vs. an restriction orifice installed on the spill-back line?

The use of a recirculation valve results in less energy usage and it reduces the volumetric flow requirement for the pump. In some cases the impact of the these variable is significant and in other cases it's not. Evaluate your case and decide. There's one other factor that sometimes comes into play, and that is plugging tendency. If it's a potentially plugging service, then a continuous recirculation flow (e.g. using an RO) is less likely to become plugged.

An FIC - FCV loop works for a reasonably wide range of differential pressures across the FCV, while the RO will only work when the dp across the RO matches the design case dp.

If your pump is operating a large percent of the time and at flows greater than minimum then an orifice is just throwing energy away. Your pump needs to be bigger to cope with the constant recirc plus the design flow. The extra cost for an arv is well worth it. If only used occasionally, low power and most of the time in standby/low flow mode then an orifice is a cheap and simple solution.

My case is a system already built but not commissioned yet, to be used for pumping waste HC and oily water (2 independent pumps) from a closed drain system drum to offsite (submergible pumps), using a continuous recirculation spill back on each going back to the drum. I am puzzled on whether these systems would not need a globe valve on their spill back lines to regulate this flow in order to ensure we can get forward flow to offsite. The spill back lines are immediately after the pump’s discharge before the check valve and the globe valve to be manipulated to send the flow towards offsite. The pumps are: Waste HC (discharge 3”x2”/ rated flow, 18 m3/hr. (minimum 12 m3/hr), Power, 16 kw); Oily water (discharge= 2” x 3” / rated flow=19 m3/hr. (minimum 9), power= 9 kw). Regarding the RO potential plugging won’t it be MORE likely to get plugged if used on a potentially plugging service?

My pumps run on intermittent service. They run whenever the material level hits a set value, so I guess an RO has been well specified. My question now remains as per the convenience of installing a globe valve on these spill backs.

the line to offsite is connected to other delivering systems, so there is a potential for some back pressure to be exerted on the line coming from the concerned pumps when they get started by the level function in the drum. I suppose that’s the reason for the recirculation flow back to the drum, to ensure the pump’s manufacturer ’s specified pump-protecting minimum flow, while the operator takes action to adjust the forward flow to offsite through the globe valve installed on the forward line.

If the spillback lines are manifolded together well before the spillback return line into the CD drum, then pressure in the return line will vary depending on the actual spillback rate, and that will affect the actual spillback flow from each pump. A globe valve on each pump discharge may work for a limited range of built up backpressures in the common dishcharge line going offsite. If you find your forward flows vary beyond the operating range for the globe valve, then would suggest a PIC - PCV on each pump discharge.

Does anyone know about a formal/official specification/engineering standard documenting or providing guidelines for selecting an specific pump recycle system? Meaning when to select: Continuous recycle (Use of ROs), Control Loop (Use of Control Valve). API 610 does not address this issue, as far as I know.

Install a backpressure control valve on the spillback line for each pump, if you have a pump Q-h curve which is not flat at around the operating point. Unless there are issues with cavitation d/s of the spillback PCV, the RO will then not be required.