Positive Diplacement Pump 3

[SENGUTTUVAN]

I would like to know when we can go in for a Plunger Pump and when we can go for piston pumps. I understand that for very high pressures Plunger pumps are suitable. Whether pumping liquid also a factor of its selection? Pl help.

 

[curtis26]

What determines this is the fluid you are pumping. Muds, slurries, sludges, or anything with lots of solids or abrasives in them require pistons. Clean fluids use Plungers. Also, anything over about 160 Deg F is going to have to use a plunger.

 

[Montemayor]

It has been my 48-yr experience that piston Positive Displacement (PD) pumps are used primarily where double-action effect is needed, required, or desired. Otherwise, a plunger pump is applied. I have used plungers in excess of 350 oF with no problems. I have used plungers at -350 oF & 3,000 psig with no problems. I have also used plungers with 10,000 psig discharge - also, no problems (& in one, single pumping stage).

The problems with plungers are usually centered on the properties of the applied plunger seals. The seals are the weakest point in the mechanical design. If the seals can withstand the temperature, pressure and lack of fluid lubricity, the application operates OK.

Any fluids with solids applied in a double-acting, piston PD pump with rings is going to ultimately fail as the rings wear out. The finer the solids in the fluid, the more accelerated the ring wear will be. I would NEVER apply a fluid with solids on a piston pump with rings. A plunger pump has no rings and, therefore, can sustain more fluid contamination. In the case of fluids with solids in a plunger pump, the valves result as the weakest operating component - with the seals being the next weakest point.

In short, my experience runs directly opposite of what curtis26 says. I restrict the application of ringed, double-acting piston pumps to max. 500 psig in one stage - as long as the fluid is 100% clean liquid and at ambient temperature. Plungers take more abuse, are more dependable (no ring wear), and can take more temperature and much, much higher pressures.

 

[SENGUTTUVAN]

Thank U montemayor. The serivce fluid here is Chilled glycol used for the chiller plant . The selection made is Plunger pump. My query now is why not Piston pump selected? It is only to understand which is best among these two.

 

[Montemayor]

Which is "best" or which is the optimum selection depends on a variety of basic data - most of which we don't have. Your query is vague and general; therefore, the responses are going to be general in nature and with some guesses.

Assuming the chilled glycol is clean and filtered, is a nominally "large" flow rate (greater than approx. 20 gpm), and is discharged at a nominal pressure of 50 to 150 psig, then I would suggest that a piston PD pump is probably the optimum selection. And that's a general response.

The above reason is primarily based on the rather benign process conditions and the fact that the piston PD pump will be less of a capital cost, $/gpm.

 

[btrueblood]

Good, succinct summary of use/application for plungers, Monte, a star for you from me, too.

 

[curtis26]

Montemayor, you should probably clarify that there are two types of pistons. 1) Rubbers and 2) Hard Rings. I was referring to the slush type rubbers in my recommendation for the slurries, sludge’s, etc. Rubbers don't have the issues with solids oand abrasion like the Hard Ring pistons and plungers both do.

The Hard Ring Pistons are used for condensates (Ethane/Methane, Liquid Propane Gas, Natural Gas Liquids, Lean Oil, etc.) and Clean Crude. The slush type rubber pistons are used for Crude w/ Solids, Drilling Mud, Fresh Water, Mine Water, etc.

The rubber pistons get soft at high temperatures which is why I said that Plungers are required for applications above 160 deg. (I think you misread this. I recommend plungers for high temperature the same as you. Your experience confirms what I said. It is not the opposite.)

The continuous duty double-acting pumps are normally only available with pistons (plungers aren't an option). They operate at speed under about 80 RPM and due to the slow speeds last for ever. These are generally used for crude oil transfer or boiler feed water. The initial cost for the double-acting pumps is higher than that of a continuous duty single-acting plunger pump that will do the same operating conditions, but they more than make up for the cost over the life-span of the pump.

The continuous duty single-acting pumps are normally only available in plungers(Pistons not an option). They are a lot cheaper than the single-acting pumps but run at speeds from 200-500 RPM (and thus don't last as long as the double-acting pumps).

The only pump models where you normally will see an option between pistons and plungers are the intermittent duty single-acting pumps. Pump style (single or double acting) and duty (continuous or intermittent) will usually determining if you get a piston or plunger, but if have whittled down to one pump model and still have an option between pistons and plungers, the pressure and flow rate will not play into the decision. Both as suitable for any given operating condition (high pressures and so forth). The deciding factor is the properties of the fluid you are pumping.

 

[SENGUTTUVAN]

Montemayor, the chilled glycol used is a clean fluid and the pump is a smaller one of 150LPH capacity at 3 bar discharge pressure. Viscosity is 1.73 . The process requirement is Plunger pump . I am querying with them the reason for selecting a plunger pump. Now after going through your explanation , i am checking up with process agressively.