A Wates pressure vessel works by using compressed air to store hydraulic energy and release water as system demand changes. It is commonly installed with water pumps and booster pump systems to maintain pressure, provide usable water drawdown and reduce unnecessary pump starting and stopping.

Understanding the Wates pressure vessel working principle is useful when selecting, installing and maintaining a pressure vessel for residential, commercial or industrial water supply systems.

This guide explains the operating cycle, air pre-charge, bladder or membrane function, drawdown and the relationship between the pressure vessel and booster pump.

Basic Working Principle of a Wates Pressure Vessel

The basic principle is simple:

Water enters the vessel → air is compressed → energy is stored → water demand occurs → compressed air expands → water is discharged.

Unlike water, air is highly compressible. A pressure vessel uses this property to create an energy-storage effect within the water supply system.

The vessel normally consists of two functional sections:

• A water chamber
• A compressed-air chamber

A flexible bladder or membrane separates the water and air, depending on the vessel design.

Why Is Air Used Inside a Pressure Vessel?

Water is practically incompressible under normal booster pump operating conditions.

If a pumping system contained only water with no effective compressible volume, even a relatively small change in water volume could cause a rapid pressure change.

Air behaves differently because it can be compressed.

When water enters the pressure vessel, the trapped air is compressed. This compressed air stores potential energy.

When water demand occurs, the air expands and forces water back into the piping system.

Therefore, the air cushion acts somewhat like a hydraulic spring.

Main Components of a Wates Pressure Vessel

The pressure vessel working principle depends on several components working together.

Vessel Body

The vessel body contains the pressurized water and air arrangement and must be suitable for the

system's maximum permitted operating conditions.

Bladder or Membrane

The flexible bladder or membrane separates the water from the compressed air.

As water volume changes, the membrane moves and allows the air volume to expand or contract.

Air Valve

The air valve is used to check and adjust the vessel's pre-charge pressure.

Water Connection

The water connection connects the vessel to the booster pump discharge or pressurized piping system.

Stage 1: Pressure Vessel Pre-Charge

Before the vessel is filled with water, the air chamber is pressurized to a specified pre-charge pressure.

For example:

Pump cut-in pressure: 3.0 bar
Illustrative vessel pre-charge: 2.8 bar

The exact pre-charge setting depends on the system design and manufacturer's requirements.

The vessel should normally be checked for pre-charge with the water side depressurized.

At this stage, the vessel contains little or no system water and the air occupies most of the available internal volume.

Stage 2: Booster Pump Starts

Water demand causes system pressure to decrease.

When pressure reaches the controller's cut-in setting, the booster pump starts.

For example:

Cut-in pressure = 3 bar

The pump begins delivering water.

Part of the pump flow supplies the immediate water demand, while excess flow can enter the pressure vessel.

Stage 3: Water Enters the Pressure Vessel

As water enters the vessel, it acts against the flexible membrane.

The membrane moves and reduces the available air volume.

Because the air is trapped:

Air volume decreases → air pressure increases

This is the fundamental operating principle of the pressure vessel.

The pump is effectively using hydraulic energy to compress the air cushion.

Stage 4: Air Is Compressed

As additional water enters:

Water volume ↑

Air volume ↓

Air pressure ↑

The compressed air now stores energy.

The relationship between pressure and volume can be approximated using the gas-law principle:

P₁V₁ ≈ P₂V₂

where:

P₁ = initial absolute air pressure
V₁ = initial air volume
P₂ = final absolute air pressure
V₂ = final air volume

For engineering calculations, absolute pressure, rather than gauge pressure, must be used.

Stage 5: Pump Reaches Cut-Out Pressure

Assume the booster system operates between:

Cut-in pressure: 3 bar
Cut-out pressure: 5 bar

As the pump continues operating, system pressure increases.

At approximately 5 bar, the pressure controller stops the pump.

At this point:

• The vessel contains pressurized water.
• The air chamber is compressed.
• Hydraulic energy is stored.
• The pump is stopped.

The system can now supply some water without immediately restarting the pump.

Stage 6: Water Demand Begins

Suppose a tap is opened after the pump has stopped.

Water begins flowing from the pressurized system.

As pressure decreases, the compressed air inside the vessel expands.

The expanding air pushes against the membrane, forcing stored water out of the vessel.

The operating sequence becomes:

Air expands → membrane moves → water leaves vessel → system pressure falls

Stage 7: Pump Restarts

Water continues leaving the vessel until system pressure reaches the pump's cut-in setting.

For example:

Cut-in = 3 bar

The pressure controller detects this lower pressure and starts the booster pump.

The vessel then begins charging again.

The entire operating cycle repeats automatically.

Complete Pressure Vessel Operating Cycle

The working principle can therefore be summarized as:

1. Vessel is pre-charged

↓

2. Water demand reduces pressure

↓

3. Pump reaches cut-in pressure

↓

4. Booster pump starts

↓

5. Water enters the vessel

↓

6. Air is compressed

↓

7. System reaches cut-out pressure

↓

8. Pump stops

↓

9. Water demand occurs

↓

10. Compressed air expands

↓

11. Water leaves the vessel

↓

12. Pressure falls to cut-in

↓

13. Pump starts again

This cycle continues according to water demand.

Understanding Pressure Vessel Drawdown

An important technical term associated with pressure vessels is drawdown.

Drawdown is the usable volume of water discharged between the pump's cut-out and cut-in pressures.

For example:

Nominal vessel volume: 300 litres

This does not mean the vessel provides 300 litres of usable water.

The vessel must always maintain an air cushion. Therefore, only part of its nominal volume becomes usable drawdown.

Drawdown depends mainly on:

• Vessel volume
• Air pre-charge
• Pump cut-in pressure
• Pump cut-out pressure

Example of Pressure Vessel Drawdown

Consider an illustrative system:

Vessel: 300 litres
Pre-charge: 2.8 bar gauge
Pump cut-in: 3 bar gauge
Pump cut-out: 5 bar gauge

For a simplified theoretical calculation, convert these to absolute pressure:

Pre-charge ≈ 3.8 bar absolute

Cut-in ≈ 4 bar absolute

Cut-out ≈ 6 bar absolute

An idealized drawdown fraction can be estimated as:

Drawdown fraction = Pre-charge absolute × (1/Cut-in absolute − 1/Cut-out absolute)

Therefore:

3.8 × (1/4 − 1/6) ≈ 0.317

Approximate theoretical drawdown:

300 × 0.317 ≈ 95 litres

So under these assumptions, a nominal 300-litre pressure vessel provides approximately 95 litres of theoretical drawdown, not 300 litres.

Actual drawdown should always be checked against the manufacturer's technical data.

Why Is Pre-Charge Important?

Pre-charge directly affects the pressure vessel's usable capacity and operating behaviour.

If the pre-charge is incorrect, the vessel may not operate through its intended range.

Incorrect pre-charge can contribute to:

• Reduced drawdown
• Frequent pump cycling
• Pressure instability
• Poor system performance
• Increased membrane stress

The required pre-charge should therefore be established according to the booster system and vessel manufacturer's instructions.

How Does a Pressure Vessel Reduce Pump Cycling?

Consider a system with no effective pressure storage.

A user opens a tap for a few seconds.

System pressure drops.

The pump starts.

The tap closes.

The pump stops.

Another small demand occurs.

The pump starts again.

This repeated operation is known as short cycling.

A correctly selected pressure vessel provides a buffer of pressurized water.

Small demands can therefore be supplied from the vessel before the pressure falls sufficiently to start the pump.

This reduces unnecessary pump starts.

Pressure Vessel Operation with a Single Booster Pump

In a conventional single-pump system, the vessel works directly between the selected cut-in and cut-out pressures.

For example:

Pump ON: 3 bar
Pump OFF: 5 bar

The vessel charges while the pump operates and discharges water while the pump is stopped.

Pressure Vessel Operation with Twin Booster Pumps

A twin booster system may use:

• Duty/standby operation
• Duty/assist operation
• Alternating operation
• Staged control

The pressure vessel provides hydraulic buffering while the booster controller determines which pump should operate.

Pressure vessel sizing must therefore consider the actual control sequence rather than simply multiplying the vessel requirement by two.

Pressure Vessel Operation with a VFD Booster System

A Variable Frequency Drive system operates differently.

Instead of relying primarily on fixed cut-in and cut-out pressure operation, the VFD changes motor speed to maintain a target system pressure.

In this type of installation, the pressure vessel may be used primarily to:

• Handle very small water demands
• Stabilize pressure
• Reduce unnecessary starts
• Absorb short pressure fluctuations
• Improve controller response

VFD pressure vessel sizing should follow the pump and controller manufacturer's requirements rather than automatically using conventional fixed-speed drawdown calculations.

What Happens If the Bladder or Membrane Fails?

If the membrane or bladder is damaged, the separation between water and air may be lost.

Depending on the vessel design, symptoms can include:

• Frequent pump cycling
• Very low drawdown
• Unstable pressure
• Loss of pre-charge
• Water coming from the air valve

A damaged membrane or bladder can prevent the vessel from performing its intended hydraulic buffering function.

What Happens If the Vessel Becomes Waterlogged?

A vessel is commonly described as waterlogged when it has lost an effective air cushion.

Without sufficient compressible air volume, very little water movement can produce a rapid pressure change.

The result may be:

Pump ON → pressure rises quickly → pump OFF → pressure falls quickly → pump ON again

This is a common reason for frequent pump cycling.

Wates Pressure Vessel Capacity

Wates pressure vessels are available in different capacities depending on the required product range and application.

Common capacities can include:

• 60 litres
• 100 litres
• 200 litres
• 300 litres
• 500 litres
• 750 litres
• 1000 litres
• 1500 litres

Vessel capacity should be selected according to required drawdown, pump flow, operating pressures and control method.

Wates Pressure Vessel Pressure Ratings

Pressure rating is separate from vessel capacity.

Depending on the product range, Wates pressure vessels may be available for different maximum pressure requirements, including:

• 10 bar
• 16 bar
• 25 bar

The correct rating should consider the maximum pressure the vessel may experience, not merely the normal system operating pressure.

Pump shut-off head, static pressure, system design pressure and potential transient pressure should all be considered.

Wates Pressure Vessel Supplier in UAE

DXB Solutions LLC supplies Wates pressure vessels for booster pump and water pressure systems in the UAE.

Different capacities and pressure ratings are available for residential, commercial and industrial applications.

Our team can assist with vessel selection based on:

• Pump flow
• Pump head
• Cut-in pressure
• Cut-out pressure
• Pre-charge
• Required drawdown
• Number of pumps
• Fixed-speed or VFD operation
• Maximum system pressure

DXB Solutions LLC serves customers in Dubai, Sharjah, Ajman, Abu Dhabi, Ras Al Khaimah, Fujairah and across the UAE. For more info contact Wates Pressure Vessel Supplier in UAE or call us at +971 4 2522966.