VFD for Pumps

Reduce discharge pipe diameter.

B V Rao,

1. The "head pressure" can be split into two parts-the pressure needed to overcome any static load (such as the vertical lift from the pump to the water level in an elevated tank), and the dynamic line losses that occur as the fluid is moved through the piping. These dynamic line losses are flow-dependent, so as the flow rate decreases, the total head pressure required will also decrease.

2. A normal design will have a discharge pressure and flow rate higher than those needed for system operation. This is to ensure capacity for future growth in needs and for degradation of system components (such as increased friction in the lines due to scaling in the pipes). A good VFD installation can use a sensor to tell if the flow rate is meeting the system needs.

For these reasons, you can apply a VFD to a pumping situation to adjust the flow rate to meet your needs. A VFD will allow the flow to meet current needs even when the system is capable of supplying greater needs, and will adjust the speed downwards to compensate for the decreased dynamic line losses that come with reduced flow rates. This is why I can recommend VFD's for many pumping applications and know that their installation will result in significant energy savings.

--JMM

For submersible pumps,THD, Total Dynamic Head is a calculation of several factors, (friction loses, depth of pump, SG of fluid, diameter of discharge pipe, etc) when working out the the well head pressure, regardless if it's a VSD or FSD(fixed speed). To reduce the fluid flow a (variable)choke MUST be introduced in the flow line, usually at the well head. This choke works by reducing the flow of fluid and as it is closed to reduce the flow the well head pressure increases and the motor current drops. Opening the choke has the opposite effect. However if an orifice plate (fixed size) has been calculated and is installed into the flow line then the flow and well head pressure would be a constant dependent of the discharge pressure of the pump, which is in turn relative to the RPM of the pump which is a result of the Hz supplied to the motor. The laws of affinity apply to the motor & pump in this case. You must also take into consideration the operating range of the pump in question. The flow rate at surface must be within the OR other wise the pump will not have a long run life, reducing the flow rate can only be within the OR window, however operating the pump on a VFD does move that OR window up or down the scale, but you can STILL operate outside the OR even if you have a VFD. Again check the laws of affinity! I don't see why you are worrying about the head losses if you reduce the flow rate as one is relative to the other and reducing the pipe size will only increase the discharge pressure of the pump, and in some cases it could exceed the pump's "burst pressure" rating and destroy the pump. One other point, a VFD can also increase the speed of a pump, as for the sensors, DHT's (downhole monitoring tools) can be connected to the VFD, they monitor pump intake pressure & pump discharge pressure (as well as motor temp, BH temp & vibration) and are usually configured to reduce speed if the BHP (bottom hole pressure) drops. This speed reduction happens so the dynamic fluid level above the pump intake is kept at a constant height above the pump intake. If you would like the formula for THD, pump loses and the laws of affinity please let me know

If you want a centrifugal pump to deliver variable flow at constant head( pressure) it is quite tricky.

Depending upon the liquid handled and actual flow/pressure requirements, you may have a choice of using a positive displacement pump that will meet this requirement very easily using a VFD.Since the flow rate is directly proportional to speed and independent of ( well almost) pressure, you can have the same discharge pressure regardles of the flow rate. No need for any elaborate pressure sensing or valves to control the pressure etc.Of course, it is understood that you have either a static head component that dictates this or a need to pump against a pressure which is constant.

Had you elaborated on the system, liquid handles, pump used with piping details one could answer in more certain terms.

Chuck Cowlagi

Ahhh.....silly me never thought about PCP (progressive cavity pump), however they are usually used in the oil industry for the VERY heavy oils, or in the food industry for pumping peanut butter and other such things. Speed control for this type of pump can be achieved with a VSD or as they are usually surface driven, by changing the size of the drive pulleys. But the VSD for PCP's is only installed to give soft start (ramped starting) and prevent drive excessive shaft wind-up and breakages.

thanks all for the feedbacks.

We do it all the time. Use a peristaltic pump and just slow it down with a VFD.

Progressive cavity pumps, air diaphragm pumps, peristaltic pumps, would reduce flow based on a VFD, or change the air pressure/flow (air pumps), or for a new-constant-target (rather than a variable target) change the pulley ratio, or the gear box ratio, or choose a different motor with lower motor rpm, etc. Gear pumps may be similar, but less sure about that.

Differs by type and supplier. Check your catalogs for pressure response and have fun.

Make sure downstream line size, partially open valve, etc is not restricting factor, as increasing flow will increase head due to friction backpressure (and vice versa).

Is there any reason for not using just a flow control valve on the delivery line?

that would depend on what type of pump you are using, it's application and a whole host of other perimeters that would have to be considered. This question cannot be answered hypothetically. In the case of ESP's as I stated in an earlier reply, the most common way to control the flow is with the choke valve at the well head, or a set orifice plate or either with a VSD/VFD. However PCP's are something different. Pump theory is not something that can be explained in this forum. If you are looking for pumps with a certain application in mind then contact the pump manufacturer and speak to their application engineer, I know they will be only to pleased to help. One last thing, a shut-off valve should not be considered as a means of controlling the flow, the flow rate though the opening will destroy the valve, it's seating and the moving valve gate in a matter of days, quicker if there are solids or sand

If you're already using a positive displacement pump, such as a gear, vane, piston or diaphragm etc. and it's being driven by a VFD or ASD then you should be able to adjust the VFDs parameters to maintain the motors full torque percentage and reduce its speed to a calculated RPM based on the pumps CIR (cubic inch per rev.) to give you the exact gpm / lpm you need and maintain maximum head.

If you're using a centrifugal pump and you need to regulate your flow but maintain full pressure, you can't use a VFD. If you slow the speed of a centrifugal pump you subsequently lower its dead-head max pressure as well. You're better off to run your motor at nominal frequency and use (depending on the amount of flow required and the fluid you're pumping) a pressure compensating flow control valve, or just a flow restricting in-line orifice and a pressure reducing valve or relief valve. If you want to vary the flow and keep a constant head pressure then you need to get into proportional or servo valves with a pressure reducing or relief valve to clamp pressure a your desired setting.

Nathan, H.S.

Could you please tell what parameter should be chosen for VFD for gear pump? Constant torque? or variable torque?

Thanks,