Check Valve Selection and Installation

A check valve operates automatically; it is not operated by a lever or an actuator.

Check valve designs are given on the Check valve types page.

Principles of selection and installation

Sizing: By flow rate, not line size

Direction: Arrow on the body in the direction of flow

Pump discharge: Check valve first, then isolation valve

Water hammer: Fast-closing (spring-loaded) design

Check valve selection criteria

Medium: liquid, gas or steam; whether it is corrosive
Maximum pressure and minimum cracking pressure
Maximum and minimum temperature of the medium
Type of connection
Size or hourly flow of the medium
Horizontal or vertical installation
Material compatibility and ease of access for repair

Cracking pressure

A minimum pressure difference between inlet and outlet is required for a check valve to open; this is the cracking pressure. Its value varies with the size and design of the valve. Selection confirms that the system can generate this pressure.

Determining check valve size

The determining factor is the operating flow rate, not the line size. In an oversized body the closing element remains partly open and chatters; this is the most common cause of premature failure in the field.

On a low-flow line, selecting a body one size smaller than the line is in most cases the better choice; the valve then operates fully open. The minimum flow rate of the line must be above the value required to open the valve fully.

Difference between a check valve and an isolation valve

A check valve is not operated; it only prevents backflow and may permit slight leakage in the closed position. It cannot be used to isolate the line. Where the line is to be isolated for maintenance, an isolation valve is used.

At a pump discharge the check valve is installed first, followed by the isolation valve.

Common errors in check valve installation

Error	Consequence
Installed against the direction of flow	The valve remains permanently closed and the line is shut completely. The arrow on the body indicates the direction of flow.
Installed too close to the pump discharge	Turbulent flow causes the closing element to chatter. Straight pipe is allowed between the pump and the check valve.
A valve designed for horizontal lines used on a vertical line	The disc does not close fully and back leakage occurs.
Operation at low flow	The closing element oscillates partly open and wears prematurely.

Connecting elements must be of the appropriate standard, pressure rating and size.
Pipe and flange axes must be aligned and the bolts tightened crosswise.
The line is cleaned with compressed air or steam before installation.

Water hammer and check valves

When the pump stops, the flow reverses and the check valve closes. If closing is not completed before the flow reverses, the returning liquid is stopped abruptly and the pressure wave known as water hammer occurs. This wave can damage gaskets, weld seams or pump seals.

Noise and shock at the moment of closing are in most cases a sign of late closing. The remedy is not a heavier element but a faster-closing design. If hammer persists at the pump discharge, soft start, slow stop or an air vessel are considered.

Sealing and fault symptoms

Symptom	Meaning
Line drains after the pump stops	Closing element not closing fully
Pressure falls slowly when the line is stopped	Foreign matter lodged on the seat; the cleanliness of the seat is checked first
Continuous chatter	Closing element oscillating
Higher pressure difference than expected	Deposits or wear on the seating surface
External leakage at the body joint	Gasket replacement required

Recurring leakage on the same body is a symptom of a problem elsewhere on the line (excessive vibration, incorrect sizing).

Spare parts and safety

Spare parts: body gasket, sealing ring, hinge pin and bush on the swing type, spring on the spring-loaded type.
Safety: cover and body bolts are not touched under pressure; before dismantling, the line is isolated and the pressure released.
Storage: in unopened packaging, in a dry place; flange covers are not removed.

For inspection intervals, see the Check valve types › Inspection and maintenance section.

Frequently asked questions

Where is a check valve installed?Is a check valve selected in the same size as the line?Why does a check valve make noise?What is done when a check valve leaks back?

It is installed where backflow is not permitted, with the arrow on the body pointing in the direction of flow. At a pump discharge the check valve comes first, followed by the isolation valve.

Not always. The determining factor is the flow rate; on a low-flow line, selecting one size smaller allows the valve to operate fully open.

Continuous chatter indicates that the closing element is oscillating. Slamming at the moment of closing is a sign of late closing, that is, of water hammer.

Back leakage is caused in most cases by foreign matter lodged on the seat; the cleanliness of the seat is checked first. On spring-loaded types, spring fatigue also delays closing.