11 replies to this topic

[A mukherjee]

Hi everyone,
Can any one explain me what is the "dry run" of a centrifugal water pump?
Thnxx in advance..

[Art Montemayor]

A "dry run" is an American expression used to describe a rehearsal or a practice exercise. I don't think that is what you mean to state.

I think what you want to refer to is having a pump operating "dry" - without any liquid being pumped due to a depletion of its suction liquid. That is called running (or operating) dry. This type of operation is very undesirable because it forces the pump to operate without any lubrication or cooling to its internal bearing parts. This can cause physical damage to a pump - particularly a centrifugal type.

[A mukherjee]

When can this kind of situation occur? Suppose a pump is constanly pumping fluid ...but due to some blockages (e.g., a blockage of control valve at discharge) it is not moving. So the discharge pressure will gradually rise. Is there any possibility of this "dry" running of a pump taking place?

[Art Montemayor]

Yes, this is a possible scenario for a process pump - particularly one that is part of a continuous process and is found out in an isolated or remote location.

The "dry running" of a pump can happen because of two reasons:

• The pump's discharge is blocked to flow;
• The pump's suction is either blocked or starved of flow.

Although this doesn't happen frequently, it can happen. It has happened to me on various occasions. The second version is the one that I have witnessed more often - particularly with centrifugal pumps.

[kkala]

An example when pumps were running dry (feed starving) has been met in a phosphoric acid plant in ca 1978. The liquids from a rotating filter (to remove gypsum) fell into hoppers, which fed centrifugal pumps without any control. Flow into the hopper was seldom balanced by pump flow, so we could see the discharge flow running, then stop, then running again and so on. Pumps were going to maintenance shop every (say) 4 days. Whifley offered pumps that could run dry according to their sayings, but we had not tried them.

[kkala]

A "dry run "pump, I haven't heard it before.
Maybe you refer to the beneficiation equipment named "Rotary dryer", [size="2"]it is used for drying materials with humidity or granularity in the industries of mineral dressing, building material.

Well, term may be idiomatic, but it means pump operating without liquid passing through it, as explained by
Art Montemayor.
By the way, name of supplier to offer pumps that could run dry without damage in 1978 (according to their saying) was Wilfley (not Whifley). But their site now does not seem to reveal such a kind of pump, at least at first look.

Edited by kkala, 31 August 2010 - 03:01 PM.

[djack77494]

There are a number of pump styles that can be designed to "run dry". Certainly our common high speed centrifugal pump would NOT be a good choice for this type of application. But something like a progressive cavity pump (e.g. Moyno) can be built to handle flow discontinuities including running dry. that's just one example; there are others. Many centrifugal pumps are designed to be "self priming". These pumps can run dry during startup as they "pump" air out of their suction lines before starting to pump liquids.

Edited by djack77494, 31 August 2010 - 03:59 PM.

[Himanshu Sharma]

@djack

Sir ,

As per my knowledge Progressive cavity pumps should always have hydrodynamic lubrication.

The pump has a stator rotating against a rubber stator.

At the points where the rotor touches the stator, the surfaces are generally traveling transversely, so small areas of sliding contact occur. These areas need to be lubricated by the fluid being pumped (Hydrodynamic lubrication). This can mean that more torque is required for starting, and if allowed to operate without fluid, called 'run dry', rapid deterioration of the stator can result.

Please clarify !

[Paktongkoi]

Dry Run terminology can also be be referred to an equipment which run without lube oil in the bearing housing. Some of the causes are lubrication line clogged, filter clogged, leaking anywhere. This will eventually lead to catastrophic failure of the equipment.

[Gause]

For centrifugal pumps running dry, is there any limit on motor size or speed that would not cause mechanical damage to pump components? For example, if a very small pump with 2 hp motor were run dry, would its seals or bearings or other components fail or be damaged? How long would it take? I imagine it is probably very case sensitive, but I'm just looking for some rules of thumb or some guidance documentation.

Same question for deadheaded condition.

Retrofitting 1960's plant for preventing hazards created by running pump dry or deadheaded resulting in seal or bearing or other component damage. Another angle may be improving reliability of equipment by protecting it against damage caused by dry running or deadhead.

Edited by Gause, 04 January 2011 - 12:03 PM.

[deltaChe]

Unless a pump is designed to run dry, it should never be allowed to do so.

The main cause of running dry is a loss of supply liquid. This could occur in (1)
an interruption to the feed into the supply tank. The way to prevent this problem is add an alarm and shut off the pump automatically once a minimum acceptable level in the tank is no longer available.

[Rama]

"For centrifugal pumps running dry, is there any limit on motor size or speed that would not cause mechanical damage to pump components?

Same question for deadheaded condition."

There is no fixed or thumb rule to determine how long a pump could run dry before it gets damaged. Several factors have a role to play in this. For e.g. some water in the sealing system might delay the damage if only the inlet is totally blocked. Dry run resulting from a partially blocked suction can take a longer time to show up. The motor size should not really matter as unless there is a case of overload, the motor just draws less power when running dry than in the normal condition, till some damage or seizure takes place. Quite often the damage starts at the seal area. A small amount of liquid in the casing can get heated up next due to churning. Bearings being lubricated independently, the damage may not occur immediately.

I am not sure what you mean by deadheaded: If you mean a blocked or closed delivery, then this is a less serious case as the pumps can run with a closed delivery for quite a few minutes, depending on the type and temperature of the fluid, before the casing gets heated up enough to cause damage. I am yet to see a pump getting seriously damaged due to a blocked delivery. However I have seen pumps running dry due to inadequate NPSH where the suction was negative (i.e. below the pump centre line.) and the temperature not properly controlled.

Ram.