Frequently Asked Questions

The PCP pump is a progressive cavity pump. This name explains how it works.

There is a free space between the stator and the rotor (cavity) which, during the rotation of the rotor, will propagate from the suction of the pump to its discharge.
The cavity will move by wrapping around the rotor and progressively move towards the exit.
This cavity contains the pumped fluid.

When the stator wears, the rotor is less tight in the stator.
When the fluid arrives at the outlet of the pump, it must overcome the discharge pressure of the circuit.
The fluid will therefore seek the path of least resistance.
If the least resistance is the seal between the stator and the rotor, part of the fluid will leak to the previous cavity.
The greater the wear of the stator, the more this leak will be important. This leak is the cause of the decrease in the flow rate of the pump.

The pump is a positive displacement pump, its volume has not changed but some of the flow has “leaked” to the inside of the pump.

The choice of the material depends essentially on 2 factors, the chemical compatibility and the resistance to abrasion.

• Chemical compatibility
  To work with chemically aggressive products, choose for stainless steel.
  The quality level of the stainless steel (304, 316, duplex) will depend on the fluid to be pumped.
  Food applications require the use of stainless steel.
• Abrasion resistance
  There are 2 ways to improve the abrasion resistance, either working with a harder material such as hardened tool steel (1.2436), or adding a layer of chrome on the rotor.
  The chrome layer can be added on any steel, but there is a risk to place it on a rotor that is not made of stainless steel.
  If the chrome layer is mechanically etched in one place, the steel will risk corroding.
  The corrosion will spread under the chrome and peel it off over time.
  And as a result, will very quickly damage the stator.

To increase the allowable outlet pressure, choose a progressive cavity pump with more stages.

In practice, this means that the number of successive cavities in the pump increases.
This results in increasing the number of stator / rotor seal lines between the pump inlet and the discharge.

Increasing the number of seal lines reduces the internal pump leakage rate at a given pressure.
This allows you to pump at higher pressures while keeping the internal leakage rate under control.

Usually, the typical standard geometry eccentric screw pump will deliver a maximum of 6 bars per stage.

When the fluid arrives at the pump outlet, it must overcome the discharge pressure of the circuit.
The fluid will therefore seek the path of least resistance.
If the least resistance is the seal between the stator and the rotor, part of the fluid will leak to the previous cavity.

The greater the discharge pressure and the greater the wear of the stator and rotor units, the greater the leakage will be.
This leak is the cause of the decrease in the flow rate of the pump.

Beyond the maximum allowable pressure of the pump, all the fluid will be leaking back between the stator and the rotor and the output flow rate will be down to zero.

Yes, the progressive cavity pumps (PCP) can be used for dosing, being a positive displacement pump.

The number of revolution of the rotor defines precisely the delivered volume, although the output pressure have an influence.
It is also a remarkable dosing pump because its flow is constant at a defined pressure, and has no pulsation, unlike diaphragm or piston pumps.

Since the pump is waterproof, it can also be used as a shut-off valve. It is therefore very easy to obtain a relatively accurate dosing, without pulsations and without the need to add a valve, just by controlling the rotation of the motor.

By increasing the speed, the flow of the progressive cavity pump (PCP) increases.
The PCP pump is volumetric. It has a given capacity that is related to its geometry.
Each rotor revolution corresponds to a displaced volume.
So the flow curve is actually a straight line directly proportional to the speed of rotation.

Note: If the discharge pressure increases or the stator wears out, the leakage rate in the pump will increase and the total flow will be lower than the theoretical flow.

In the vast majority of cases, the rapid destruction of the stator is related to a dry running of the pump.
That is, there was no fluid flowing in the pump.

Note that in an eccentric screw pump or progressive cavity pump, the stator rubs continuously on the rotor.
It is an elastomeric (usually rubber) in contact against metal.
If the contact is direct, without fluid to lubricate the friction becomes important and consequently, the stator and rotor temperature will increase rapidly.

No elastomer is resistant to very high temperatures (see what temperature for which elastomer?).
Beyond its temperature limit, the stator will become mechanically damaged.
As a consequence, the rotor and stator will not be sealing between pump inlet and outlet, the cavities in the pump will enable leakage and the PC pump will no longer be pumping.

The other rarer cases of less rapid destruction are: the chemical or mechanical attack of the pump.
If the pumped fluid suddenly changes composition, or a new fluid is pumped, rapid wear can occur.

As example, the sudden presence of sand in process water or a new cleaning product for food pipes are typical cases.

Yes and no.
As the cavities between the rotor and the stator are sealed, the pump will be able to empty the air of a suction pipe, so it is indeed self-priming.
 
But the eccentric screw pump hates dry running. This very quickly deteriorates the stator.
So if you start a dry pump, you will achieve the desired effect at the cost of the deterioration of your pump.

But if you usually pump oil or fuel and the stator stays fat between operations, the PCP pump will be self-priming in a sustainable way.

An eccentric screw pump is reversible, but it has a preferred operating direction: with the discharge through the free end of the rotor which is the opposite side to the motor.

By having the motor rotating in the other direction, the pump will suck from the end and pump back through the pump body.
This should only be used for low discharge pressure applications.
Indeed, in this configuration, the discharge pressure is applied on the mechanical seal of the pump.
It is therefore subject to greater constraints and this reduces its life.

The choice of elastomeric material depends on a combination of factors specific to your application:

• The temperature of the transported fluid
  Between 0 and 60°C, all work.
• Food compatibility
  There is an unlimited number of rubber compatible with food standards, namely NBR, EPDM and some Viton®. They exist in black or white versions.
• Chemical compatibility
  It must often be checked on a case by case basis.
• For non aggressive applications (biogas, water treatment, food products …), Nitrile is the reference material used for stators, it alone covers 80% of the market.
• For low temperature applications (below 0°C) or light acids and bases, EPDM will generally be a good choice.
  On the other hand, EPDM is totally prohibited in the presence of mineral oils.
• For aggressive chemical applications or at high temperatures, in most cases the Viton will be appreciated for its outfit.
  But it has its limits, such as low resistance to water vapor and caustic soda (NaOH).
• The physical properties of the stator material
  As example: abrasion resistance, where the Viton will be poor, the NBR is usually a good compromise, and for extreme cases, polybutadiene (Buna CD) is recommended.

In all cases, an analysis is required and we are ready to help you with that.
If in doubt, contact us via email at info@sps-pumps.com or via phone at +32 2 657 23 53.

The maximum output pressure will depend on the number of stages of the pump

For most manufacturers, a pump stage is 5 to 6 bar maximum pressure. A two-stage pump can thus reach 10 to 12 bars.
These are theoretical values that are measured with water and a new stator.
It is usually a sign that the stator is worn if the pump does not reach the required pressure.

Note that constant thickness stators allow to reach a double pressure (up to 12 bar per stage).

In general, it is not necessary to change the rotor each time the stator is replaced.

With the exception of very abrasive applications, the eccentric screw pump rotor should be replaced after 3 or 4 stator replacements.

Although, it is advisable to change the rotor as soon as it presents marks or if its section is no longer circular, because, in this case its wear decreases the performance of the pump or may prematurely damage the stator.

The stator of a Eccentric Screw pump can wear out too fast because of two things: speed and pumped material.

The faster we pump an abrasive material, the faster the stator will wear out.
So, depending on the material you are pumping, you have to find the ideal rotor revolution speed.

Increasing the rotor revolving speed, will result in faster wearing of the stator.

For each pumped fluid, it is important to estimate the appropriate speed.
Feel free to ask us more on this subject.

As an example, 180 rpm should be seen as a maximum revolving speed for sludge and abrasive materials.

In some cases, it it highly advisable to buy a bigger pump that will run at slower speed.
Indeed, the difference between the new pump purchase price will easily offset the costs of multiple stators replacements.

Understanding the NPSH is imperative when choosing a pump.

The NPSH is the abbreviation of Net Positive Suction Head.
In other words, it is the required water level (or pressure) by the pump to avoid cavitation (implosion steam bubbles).

For a pump to work properly, it must have a certain pressure at its inlet, which is also a function of the pump model.
This minimum required pressure is the required NPSH.
To know the NPSHr of a pump, simply refer to the pump curves.
Note that the NPSHr varies depending on the flow.
Check the point of use of your pump and plan for enough safety, as least thing you want is having your pump risking cavitating.

The piping system that feeds the pump offers the available NPSH or NPSHa.
It must be higher in the entire pump use range, than the NPSHr.
A good approach is to ensure a reserve of minimum 10% between the NPSHr and the NPSHa, with a minimum of 1 meter.

To calculate the NPSHa, it is necessary to take into account the atmospheric pressure (which varies according to the altitude), the density of the pumped fluid, the pressure drop at the maximum flow rate predicted in the supply pipes from the storage tank, as well as the vapor pressure.

The proper operation of a pumping system requires accurate calculation.
Do not hesitate to get advice before making your choice.

More on the NPSH on Wikipedia

Actually, a pump does not generate a outlet pressure. It simply move fluid or generates a flow. The pressure increases only by the resistance offered by the pipe system it feeds.
In effect, it fights the pressure drop, back-pressure of the downstream circuit.

If the pump is connected to an completely open tap, the pressure at the pump outlet is zero.
If an outlet valve is closed, the pressure will be the maximum value the pump can provide.

As an example, if there is not enough pressure at an injector end, it is because the injector is either too large or there may be too many injectors, or the viscosity is too low, … resulting in a too high flow and consequently a lower pressure.
In other words, there is not enough head loss in the circuit, so the pressure does not rise.

On a closed pump, the easy way is to calculate the ratio between the length en diameter of the stator.
This will give an indication. If the ratio is between 2 and 2.5, it will probably be a single stage eccentric screw pump.
If the ratio is between 3.5 and 5, it will probably be a two stages pump.

Careful, there are a few exceptions to this, as example for dosing pumps for which the ratio can be rather different, or for extended single stage pumps for which it is only possible to know by opening the pump and counting the cavities.
One stage corresponds to 2 complete sinusoids, or 2 cavities (one cavity being equivalent to a hollow and a hump).