How to Set the Service Water Flow on a Liquid Ring Pump

Table of Contents

• The Role of Service Water
• Too Little or Too Much Water? Balancing Two Risks
• Starting From the Manufacturer's Value
• Step-by-Step Adjustment Method
• Reading the Discharge Water Temperature
• Recording the Setting

On a liquid ring vacuum pump, service water is the liquid that forms the compression ring and carries heat away from the pump. When its quantity is set correctly the pump runs in balance; when it is set wrongly, both vacuum falls and water is wasted. This article explains step by step how to determine and adjust the service water quantity.

The Role of Service Water

In a liquid ring pump, water forms a rotating ring inside the casing. The impeller vanes work within this ring, and gas is compressed and discharged by this motion. The same water also absorbs the heat created by friction and compression and removes it from the pump through the discharge.

Service water is therefore not an auxiliary item but part of the working principle of the pump. Its quantity directly affects both the vacuum performance and the temperature of the pump.

In a liquid ring vacuum pump the service water is an indispensable fluid that provides both sealing and cooling of the pump. The rotor turning inside the pump compresses together with this water ring and creates vacuum in the system. The water lubricates the inner surfaces of the pump, lowers the temperature and prevents air from leaking through gaps. For this reason the quantity of service water directly determines the performance of the pump.

The job of the service water is not limited to being a coolant. The water ring absorbs part of the compressed gas and keeps the heat balance inside the pump. It also carries the small particles that come with the gas and prevents them from building up inside. This multi-purpose task makes it necessary to supply the water in the right quantity and at the right temperature.

Keeping a constant flow in the service water line is a basic condition for the balanced operation of the pump. Changes in the flow affect the water level inside the pump and therefore its vacuum performance. For this reason a flow meter or at least a control valve should be fitted in the service water line.

The quality of the water also affects the job of the service water. Hard water can cause lime to build up inside the pump, and over time this deposit blocks the inner surfaces and the discharge lines. Dirty or sandy water erodes the rotor and the shaft seals. The quality of the water source should therefore be one of the first things assessed in the design of the line.

Too Little or Too Much Water? Balancing Two Risks

When service water is reduced, the ring cannot be maintained properly and the vacuum the pump can reach falls. Heat removal also becomes insufficient, and the casing temperature rises. Running for long periods under these conditions can harm the mechanical seal and internal parts.

On the other hand, excessive water use raises energy and water costs. More water than necessary can create an unnecessary load inside the pump and cause problems in the discharge line. The aim is to find a balanced quantity that is neither too high nor too low.

The question of too little water or too much water is the most critical point of the setting. When too little water is supplied, the pump overheats, the vacuum falls and the internal parts can be damaged by dry running. When too much water is supplied, energy and water consumption increase, and excess water can build up inside the pump and overload the motor. The right balance lies at a point between these two risks.

The signs of the risk of too little water are a rising temperature of the discharge water, a slow build up of vacuum and a change in the sound of the pump. These signs show that the water flow is insufficient and need immediate action. If noticed early, the problem can be solved with a simple valve adjustment.

The signs of the risk of too much water are an excessive flow in the discharge line, a rise in motor current and a vacuum that falls more than expected. A pump working with too much water is under an extra load to discharge the water. For this reason excess water is a wasted resource in terms of both energy and water consumption.

A balanced setting is recognised by the vacuum value and the discharge water temperature staying within reasonable limits under every working condition. Finding this balance requires that the values given by the manufacturer and the measurements on site are assessed together.

Starting From the Manufacturer's Value

Before starting any adjustment, read the service water flow and inlet pressure recommended by the pump manufacturer from the operating manual. This value is set according to the pump type, its size and its working vacuum. Use it as the starting point and record the values during the first run.

If the process conditions in the field vary, staying within the range recommended by the manufacturer is the safest way to adjust. Before going outside that range, consult the manufacturer or an authorised service team.

The service water flow value given by the manufacturer is the starting point of the setting. This value is determined for the nominal working conditions of the pump, and if the real conditions on site are different, an adjustment is needed. The catalogue value gives a reliable reference for making the first setting.

When using the manufacturer's value, the water pressure and temperature on site must also be taken into account. A flow given on a low pressure line may not reach the catalogue value. Hot water can also reduce the efficiency of the pump. Each of these conditions affects the starting setting and determines how the first value should be interpreted.

After the starting setting has been made, the values should be watched closely during the first few hours of the pump. Vacuum, motor current, discharge water temperature and the sound of the pump should be recorded during this period. The first observations show whether the setting is correct and in which direction the necessary corrections should go.

The gradual setting method recommends changing the service water flow in small steps rather than in large sudden changes. After each step enough time should be allowed for the pump to settle at its new balance point. This patience makes sure that the setting is stable and reliable.

Step-by-Step Adjustment Method

Make the adjustment in small steps. After each change, wait a few minutes for the pump to settle and read the vacuum value, discharge water temperature and motor current. Making a large change at once makes it harder to understand how the pump is responding.

When adjusting with a valve, the flow can be read directly if a flow meter is fitted. Without a meter, you can estimate the flow by collecting the water volume over a set time in a container.

In gradual setting it is important to open and close the valve only slightly and to record every change. A big change can upset the balance of the pump and require a new setting, which wastes time. Moving in small steps makes it clear which change produced which result.

One advantage of the gradual method is that it reveals by itself the range in which the ideal flow lies. The vacuum value and the discharge water temperature together with the flow form a curve. The most efficient part of this curve shows the range to be targeted for continuous operation.

During the gradual setting it should be made sure that the pump never runs dry at any stage. When closing the valve, the water flow should not be cut off completely. Even a moment of dry running during adjustment can cause damage to the internal parts.

The temperature of the discharge water is one of the most important indicators of the service water setting. The discharge water temperature reflects the heat balance inside the pump. If the temperature rises the water is insufficient, and if it falls the water may be excessive. For this reason the temperature measurement is a parameter that must be followed continuously.

Reading the Discharge Water Temperature

The temperature of the discharge water is one of the best indicators of the balance of service water. If the discharge temperature rises over time, either the water quantity is insufficient or scaling may have formed in the heat exchange section. The difference between inlet and discharge temperature gives a view of the health of the operation.

Record the discharge water temperature together with the inlet value and track the difference between the two. A difference larger than expected shows that the flow setting should be reviewed.

To read the discharge water temperature correctly, the temperature sensor must be in the right position. The sensor should be placed at a point in the discharge line where the flow is mixed and even. A sensor in a wrong position shows values different from the real temperature and leads to a wrong decision.

The relation between discharge water temperature and vacuum level gives very valuable information about the state of the pump. A rising temperature together with a falling vacuum usually points to insufficient water flow. A falling temperature together with a falling vacuum can have a different cause, for example a blocked line.

The colour and smell of the discharge water should also be observed. A change in colour may show oil, rust or process residues coming out of the pump. These signs give early warnings about the condition of the internal parts and guide the maintenance planning.

Recording the setting creates the operating history of the pump and gives a reference for future adjustments. The record should contain the service water flow, the vacuum value, the discharge water temperature, the motor current and the date of the setting. This information helps to decide how the setting should be updated when the seasons change.

Recording the Setting

Every adjustment should be recorded with the date, valve position, measured flow, inlet pressure, discharge temperature and vacuum value. These records are used to decide how the setting should be updated during seasonal changes or process changes.

Settings should also be communicated to the shift supervisor. If different people run the same pump with different settings, unexplained fluctuations in the vacuum value can occur.

Keeping the records in order makes it easier to understand what changed when a problem appears. If the values with which the previous setting was made are known, the source of the problem can be found much faster. A setting made without a record turns into guesswork.

The setting record can also be used to monitor water consumption and energy use. When the amount of service water and the energy consumption are followed together throughout the year, saving opportunities appear. These data are also valuable for reporting the environmental performance of the plant.

Finally, the operators must also be informed so that the setting can be carried out on site. The operators should know which flow is accepted as normal, at which values action is needed and how the records are kept. This knowledge makes sure the setting is maintained without changing from person to person.