A pressure vessel is one of the most frequently underspecified components in automatic pump systems, yet it has a greater impact on pump longevity and system performance than most buyers appreciate. In a borehole, booster, or irrigation pump installation, the pressure vessel acts as a buffer between the pump and the demand on the system, storing a reserve of pressurised water that allows small draw off events to be met without starting the pump, and smoothing out the pressure fluctuations that occur as demand varies.
When correctly sized it extends pump service life significantly, but when absent or undersized, it is one of the primary causes of premature pump failure through excessive cycling.
This guide explains how pressure vessels work, the types available, and how to choose the right capacity for a given system.
What a Pressure Vessel Does
A pressure vessel contains an internal rubber bladder or diaphragm that separates the vessel into two chambers: a water side connected to the pump system pipework, and an air side pre-charged to a set pressure. When the pump runs, it fills the water side of the vessel, compressing the air charge and storing energy as pressure. When a tap is opened or demand arises, this stored pressure drives water to the outlet without the pump needing to start immediately. The pump only starts again when vessel pressure drops to the pressure switch cut-in point, at which point it refills the vessel and stops again at the cut out pressure.
The practical result is that small, intermittent demands, a brief run of a tap, a toilet filling, a short irrigation cycle, are met entirely from the stored volume in the vessel without the pump starting at all.
Only sustained demand that exceeds the vessel's storage capacity causes the pump to run. This is the core mechanism by which a correctly sized pressure vessel protects the pump from short cycling.
Why Short Cycling Damages Pumps
Every pump start draws a surge of current through the motor windings, known as the inrush current, which is typically five to seven times the pump's normal running current. This inrush generates heat in the windings and creates mechanical shock in the pump assembly. A pump that starts and stops many times per hour sustains this thermal and mechanical stress repeatedly, accelerating wear on motor windings, bearings, seals, and contactors in the control panel.
In a borehole or booster pump system without an adequate pressure vessel, even a brief draw off event such as a toilet filling can cause a full pump start and stop cycle. In a property with normal household demand patterns, this can mean dozens of short cycles per hour, each one contributing to accelerated wear. The right pressure vessel, sized correctly for the system, reduces cycling frequency to an acceptable level and extends the operational lifespan of the pump considerably. The pressure vessels range covers CIMM vertical and horizontal vessels from 3 litres to 1,000 litres to suit systems from small domestic installations through to large agricultural and commercial applications.
Types of Pressure Vessel
Pressure vessels are available in vertical and horizontal formats, and in a range of capacities suited to different system sizes:
- Vertical pressure vessels: the most common format for smaller domestic and light commercial installations; they take up a compact floor footprint and are the standard choice where space is not a constraint and capacity requirements are in the 3 to 24 litre range
- Horizontal pressure vessels: better suited to installations where ceiling height is limited or where a lower profile is preferred; available from 24 to 300 litres and often used in larger domestic, irrigation, and light industrial systems where the vessel needs to be positioned under pipework or in a confined plant room
- Vertical vessels with legs: the appropriate format for larger capacity requirements from 50 to 1,000 litres; the leg mount raises the vessel off the floor and facilitates the pipework connections needed for larger flow rates; used in agricultural irrigation, commercial water supply, and industrial booster pump installations
How to Size a Pressure Vessel
Pressure vessel sizing is the most important specification decision in the selection process, and the most commonly done incorrectly by choosing a vessel that is too small. The correct vessel size for a given system depends on three factors: the pump flow rate, the pressure switch differential (the difference between cut in and cut out pressures), and the desired minimum number of pump starts per hour.
A useful rule of thumb for domestic borehole and booster pump systems is to allow a minimum vessel capacity of approximately 10 to 15 litres of usable drawdown volume per cubic metre per hour of pump output. However, the precise calculation method published by vessel manufacturers, which accounts for the specific cut in and cut out pressures of the pressure switch, gives a more accurate result and should be used for any system where reliable pump protection matters. The general principle is clear: when in doubt, size up. A vessel that is larger than the minimum calculated requirement provides better pump protection and incurs no operational penalty.
Pre Charge Pressure and Maintenance
Every pressure vessel is supplied with a factory air pre charge on the air side of the diaphragm, typically set to approximately 1.5 bar or to 0.5 bar below the pump cut in pressure. This pre-charge must be confirmed and, if necessary, adjusted before the vessel is installed, as an incorrectly set pre charge prevents the vessel from operating within its intended pressure range and reduces the usable drawdown volume available to the system.
The air pre charge should be checked periodically with the system depressurised and the water side drained. If the pre charge has dropped, the air side can be topped up using a standard tyre pump or compressor via the Schrader valve. If water emerges from the Schrader valve when the system is depressurised, the internal bladder or diaphragm has failed and the vessel must be replaced. A failed bladder is also the most common cause of rapid pump cycling in an existing installation, and replacing the vessel is typically the first corrective action to take when cycling frequency increases without any other apparent change in the system. For systems where the vessel is associated with a borehole, the borehole pumps range covers replacement submersible pumps across all standard UK casing sizes. For booster system applications, the booster pumps range covers surface-mounted options for domestic and commercial pressure boosting duties.
Frequently Asked Questions
How do I know if my pressure vessel has failed?
The most reliable indicator is a pump that cycles on and off very rapidly with minimal water usage; confirming the diagnosis by pressing the Schrader valve on the air port with the system depressurised will show water rather than air if the internal bladder has ruptured.
What size pressure vessel do I need for a domestic borehole?
For a typical domestic borehole pump delivering 2 to 3 cubic metres per hour, a vessel in the 24 to 50 litre range is generally appropriate; the precise requirement should be calculated from the pump flow rate and pressure switch differential rather than estimated.
Can a pressure vessel be installed horizontally or vertically?
Both orientations work correctly provided the vessel is designed for the intended position; horizontal vessels are specifically manufactured for horizontal installation with the connection port in the appropriate position, and should not be used vertically.
How often should the pre-charge pressure be checked?
Checking the pre charge annually is good practice; it should also be checked whenever pump cycling frequency increases noticeably or when the system is recommissioned after a period of shutdown.
Final Thoughts
A pressure vessel is a straightforward component that does a critical job: protecting the pump from short cycling and providing a reserve of pressurised water that smooths system operation and reduces demand on the pump motor. Correctly sizing the vessel for the pump's flow rate and the system's pressure switch settings, confirming the pre charge before installation, and replacing the vessel promptly when the bladder fails are the three maintenance actions that deliver the greatest return in pump longevity.
Browse the full range of pressure vessels and pump system accessories at Henry Pumps to find the right capacity for your borehole, booster, or irrigation system.