Number of Seals per PD Pump

OK. I found the following on wikki,

"Twin Screw (Positive Displacement) The twin screw pump is constructed of two intermeshing screws that force the movement of the pumped fluid. Twin screw pumps are often used when pumping conditions contain high gas volume fractions and fluctuating inlet conditions. Four mechanical seals are required to seal the two shafts."

From this statement looks like two seals per shaft. If so, why centrifugal pumps that have one shaft have only one seal and not two.

This might be very dumb question. However, my knowledge of pump is pretty much nothing. And all the articles I have found so far, doesn't do good job of engaging me.

PD pumps can produce much higher pressures than centrifugal pumps. Maybe that has something to do with the dual seals.

Some reasons for dual seals:

High pressure differential

Hazardous fluid being pumped

Valuable fluid being pumped

In these cases, leakage through the "inner" seal (which is usually the one to leak first) is backstopped by the second seal. Also, the space between the seals may be charged with oil or other nonreactive fluid, and can be monitored for leakege before the pumped fluid itself actually leaks. This is very common in ammonia refrigeration. It is also useful for controlling fugitive emissions.

Some of the earlier posts are correct,

I like to add one more, A extra seal may be in place for is for sanitary conditions, to meet standards.

p911

Another possibility is if the pump is Diaphragm type PD pump, it may be a type called Hydraulic diaphragm pump.

With normal diaphram pumps there is only a single seal (around the diaphram), between the process fluid and the piston.

With Hydraulic diaphram pumps, there is a hydraulic fluid between the piston and the process fluid. Therefore has two sets of seals, one for each of two diaphram. One between the process fluid and the hydraulic fluid. Another between the Hydraulic fluid and the piston.

example is http://www.dencilpumps.com/diaphragm-pump.htm

Alternatively they could be inlet and outlet non-return valve seals.

Another use (of course not for pumps, but for other equipments that we make) -

one set of seals to stop in service oil inside from leaking out, the outer seal to stop ingress of dust / moisture/ any other foreigner .

usually these seals are efficient for one side stopping and allows almost free flow from other side (like the diode ).

In certain cases the chamber between seal is kept pressurised as added precaution.

Taking your questions one at a time:

1. "Do all pd (pumps) have multiple set of seals?" No. The most common types, rotary pumps (gear, screw and other less common types) usually have just one seal where the driving shaft enters the pump housing. In these pumps internal loads on pumping elements are supported by plain or antifriction bearings that are lubricated by the fluid being pumped.

2. "What requires more than one set of seals?" This is usually a pump that is specially designed to isolate the load supporting bearings and their oil or grease lubrication from a pumped fluid that is a poor lubricant, usually due to low viscosity, the presence of solid abrasives, chemical incompatibility or the need to prevent contamination of the fluid. If the pump by its nature has more than one shaft with bearings that have to be isolated then there will be more than one seal. More common in the 20th century than today are a class of large twin screw positive displacement pumps used for volume applications like cargo handling. These typically are hydraulically balanced and have bearings outside of seals that carry thrust loads, timing gear loads and secondary loads. So such a pump would have 4 seals. There are also some small sanitary stainless steel rotary pumps used in food processing that have their rotors in the middle of two bearings and therefore have two seals.

There is another arrangement often found in centrifugal chemical pumps that handle toxic or flammable liquids. This is the "double seal" that has a cavity between two seals on the same shaft that will collect any leakage of the dangerous fluid in the pump casing, dilute it with a neutral fluid flowing through the cavity between the seals and allow it to be carried away to proper disposal or recycling.

The twin screw pump mentioned in your second post is the same type as I mentioned above. These are fundamentally different from centrifugal pumps. I think some study of diagrams and operational principles of each will make the reason for the numbers of shafts fairly obvious. To detail this out for you in this forum would amount to a short course in pump design. This is something you probably need to do yourself.

Ed Weldon

Only a twin screw design out of all PD pumps require 4 mechanical seals,No other pump needs 4 seals.Here the number 4 signifies the sealing locations.4 double seals may be needed - this should not be equated to 8 seals!!

Reason : There are two shafts supported by bearings at either ends.In those designs where Bearings are placed outside the pumped fluid - 4 seals are must.Each end of each shaft needing one seal.

For designs having bearing inside the pump but not lubricated by the pumped fluid,the bearings are protected by mechanical seals - again 4 in number.

These seals are on the suction side of the pump and not subjected to discharge pressure at any time.The seal may be a single type or double type depending upon what is being pumped.But the there are 4 locations which need sealing of shaft that come out of the casing.Refer to a cross section or cutaway view of such pumps and it will be clear.Here are some of the well known manufacturers whose site will provide you the required information : Bornemann,Leistritz,Houttuin,Warren,Sier Bath plus a host of Chinese, Japanese and Russian manufacturers.

Chuck Cowlagi