Centrifugal Monoblock Pump

A centrifugal mono block pump is designed to operate with water or another suitable liquid inside the pump casing. When the pump runs without sufficient liquid, it is said to be running dry. Although the motor may continue to run normally, the pump itself can be damaged because the liquid that normally provides cooling and lubrication is no longer available.

In this article, I will explain what happens when a centrifugal mono block pump runs dry, why the mechanical seal is particularly vulnerable, and how simple protection methods such as a float switch or flow switch can help prevent dry-running damage.

Centrifugal Monoblock pump is assembly of two units; pump & motor which seems like a single unit that’s why called “Monoblock“.

This pump takes water in from the center then impeller moves the water in a curved path & push the water away from the center that’s why; centrifugal. In Total, full name is ‘Monoblock Centrifugal pump‘.

3.7 KW Monoblock pump — 3.7KW Monoblock Pump

Water pumps are used to transfer water from one place to another. We should choose the pump according to our need of ‘Discharge & Head’.

WHERE :

Discharge means rate of flow of quantity of water. (Liters per minute)
Head means total vertical height from the lowest water surface to the point of delivery. This also includes losses in pipes & fittings. This term ‘Head’ is well explained in Installation Diagram below.

7.5HP Centrifugal Monoblock Pump By LUBI

Installation Diagram for Monoblock Pump

In installation diagram below. Head is sum total of Hs + Hd + losses in pipe & fittings.

Where :

Hs is the distance from lowest point of water to pump center.
Hd is the distance from pump center to the point of delivery.

Why pumps should not be run dry ?

When a centrifugal monoblock pump is running, the water inside the pump does more than just get pumped to the discharge line. It also helps cool and lubricate certain components, particularly the mechanical seal.

If the pump continues to operate after the water supply has stopped, the impeller may keep rotating while there is little or no liquid available inside the pump casing. This is known as dry running.

The main problem is the mechanical seal. Under normal operation, the liquid around the seal helps remove heat and provides the conditions required for the sealing faces to operate correctly. During dry running, this cooling and lubrication are lost. Heat can build up rapidly at the seal faces, which may cause the seal faces to become damaged, distorted, or cracked.

As the seal deteriorates, water can eventually start leaking from the pump. In severe cases, continued operation can also damage other pump components.

Therefore, a motor continuing to run does not mean that the pump is operating safely. A dry-running pump can appear electrically normal while mechanical damage is developing inside it.

Damaged mechanical seal also dangerous for winding of induction motor. Leaking water can get inside the motor’s winding through rotating shaft That’s why pumps shouldn’t be run dry.

How to save a pump from dry run ?

By running the pump through a feedback command, such as feedback from water flow, Pump can be saved from dry run.

Next, we will explain two methods to save the pump from dry run in which first method is ‘flow switch method’ & second method is ‘float switch method’.

Dry run protection with flow switch

Dry run refers to no water in outlet/delivery pipe which means no flow in delivery pipe. By installing flow switch on delivery pipe, Pump can be prevented from dry run.

“The flow switch consist of limit switch which carry three points; Common, NC & NO. This flow switch work on mechanical mechanism whenever water pressure flows; it pushes the limit switch which further changes its electrical contact’s state.”

Mechanical system automatically goes back to its normal state when flow get zero; because of spring mechanism. Along-with it Parallelly, Relay also goes back to its normal state where NC get opened & NC contact get closed.

Electrical Wiring Dry run protection with flow switch

We are describing here electrical wiring for Monoblock pump which is running on DOL Starter(Explanation about DOL Starter).

Overall wiring for DOL starter remains the same, We just need to add single NO contact of flow switch in the circuit. NO contact of flow switch closes only when pressure flows through the delivery pipe.

At initial stage; When pump is at rest no water at delivery pipe; the NO contact of flow switch remains open. We can’t use this NO contact at the time of start. According to the demand, this NO contact must be linked to the Holding of the contactor. So that at the time of start, When we press the Start(No) button; pump get started without considering flow switch but Holding works only when there is water in the outlet. Which results while running if somehow pump looses water, the power to pump also goes off simultaneously.

Electrical Circuit for Dry Run safety with flow switch for Monoblock pump

As shown in Image above, power terminals R,Y,B & Motor terminals are already connected to the contactor.

(NOTE- Make sure to add compatible thermal overload relay & Back up fuses/MCB with the contactor !)

Since contactor’s coil voltage 230VAC, Neutral is directly connected to contactor A1 & a Control phase first passes through the NC contact of thermal overload relay than it passes through the NC (STOP) & Then passes through the NO(START). Holding supply is taken just after the NC (STOP). Before connecting Holding supply to the contactor’s NO, this supply must be passes through the NO of flow switch. At last, A2 of contactor connected to NO(START) & NO(HOLD).

Dry run protection with float Switch

“Float switch is a hollow plastic chamber which contains mechanical lever arrangement with changeover relay & a metallic ball. A cable is connected to changeover relay & brought out of the plastic chamber. This whole floatable plastic chamber is IP68 ingress protected so that no water leakage takes place inside the chamber. According to the position of float switch, changeover relay changes its contacts.”

Bisected view of Float switch from inside — Float switch from inside

A weight is connected with Float switch in such a way that when water level goes up beyond the level of Float switch; float switch get lift in UPWARD Direction. Metallic ball present inside the hollow chamber get shifted opposite to the lever which results relay’s contact remains in normal initial state.

While in hanging position, the metallic ball present inside the hollow chamber presses the lever & in turn; lever presses the limit switch Which results limit switch’s contact get changed. Which is clearly explained with the help of images below.

Float switch position high water(Left) & Low water(Right)

Float switch's Changeover relay position — Float switch’s Changeover relay position

Electrical wiring dry run protection with float Switch

Here we are taking an example of 3.7KW- 3phase Induction motor running with DOL Starter.

Since Contactor’s coil voltage is 230VAC; neutral is directly connected to the coil.
Control Phase which is connected to contactors coil should be pass through the NC contact of float switch than through NC (Stop push button) than through NO (Start push button).
For contactors holding. NO contact of contactor should be connected in parallel with NO(Start push button).

Control circuit for dry run safety with float switch

Conclusion

The post explains the functioning of Centrifugal Monoblock pumps, stressing the importance of not running them dry.

The Monoblock pump, which combines motor and pump, relies on water flow to keep the impeller and seals cool.
Running the pump dry generates frictional heat, risking damage to the mechanical seals and leading to potential leaks and motor failure.
To protect against dry runs, two methods are proposed: flow switch and float switch.
The flow switch detects the absence of water flow, while the float switch reacts to water levels in a tank, both ensuring the pump operates safely.

Dry run is prohibited in some pumps like Monoblock pump, Because these pumps get damaged under dry run condition.
From dry run protection, We can arrange a flow switch on delivery pipe which provides signal whether pressure flowing or not.
From dry run protection, We can also arrange a float switch in the water source tank from where pump is sucking the water.

Hope! You like the post. If you have any doubt, suggestion or query please do comments !

Tagged Experiences, Pumps

Centrifugal Monoblock Pump Dry Running: Risks, Damage & Protection