What makes NAF-Check the best non-return valve in its class?
Consider the following design features which have made NAF-Check the first choice among discerning engineers.

BODY - The circular wafer type body is designed to be clamped between adjacent pipe flanges and matches most international standards within the respective pressure ratings, e.g. ANSI 150/300 or PN 25/40/100. The wafer body is compact, lightweight and economical especially where expensive raw materials are concerned. Small face to face dimensions enable NAF-Check to be fitted into the most cramped of piping layouts. An integrally cast-in lifting eyeand flow direction arrowfacilitate handling and installation and minimises down time if valves have to be removed from the pipeline.

DISC - Of lightweight, low enertia shape, designed for maximum strength and rigidity. Semi-balanced by the positioning of the stub shafts, it is able to open at low pressure differentials. NAF-Check opens fully at water velocities as low as 0.6m/sec, thereby overcoming "disc flutter" commonly found in conventional valves and which leads to rapid wear of the shaft and bearings and the inevitable falling-off in efficiency.

The disc is self-centring and sealing is effected by accurately turned and bevelled seats in the body and on the disc edge, giving tight shut-off at both high and low pressure differentials.

The oblique seating angle, 15° on valves to 450mm diameter (10° for valves 500,, and above) gives a short angle of swing from fully open to close.

STUB SHAFTS - A small but important element in the design of the NAF-Check which so often is the cause of failure in conventional valves. These heavy stub shafts are positioned to give maximum strength with minimal clearance between the body and disc so that there is no unsupported length of shaft. All loads on the shaft are therefore carried in shear and not in a combination of bending and shear which can be the cause of so many failures in conventional valves used on steam and gas lines which are subject to rapid opening due to high velocity.

The stub shafts have seal welded covers on the outside of the body to eliminate all possible leakage to atmosphere, making NAF-Check an ideal valve for hazardous or "fire safe" installations.

Why should a check valve close quickly?
The answer is, of course, to eliminate reverse flow but before a check valve can close all forward flow must have ceased completely. A quick reversing flow, as found in pump systems where one or more pumps feedinto a high pressure main, requires that the valve be very fast acting indeed if a large reverse flow is to be avoided. This loss is in itself a disadvantage but the resultant "water hammer" caused when the check valve does close has a far more serious implication. Under extreme conditions whole pipe and pump systems can be wrecked and even under less arduous conditions the cumulative effect of the shock waves can impair the proper working of the system and lead to an early breakdown with the subsequent high maintenance costs that this implies.

A usual method to reduce "water hammer" casued "by slow closing" check valves is to install dashpots to slow down the movement of the disc immediately before the seat is reached. The consequent high return flow must be accepted and the arrangement will be complicated and relatively expensive. It can also subject the disc and shaft assembly to excessive stresses when eventually the disc does close-off against the higher velocity return flow.

A better way is to make the natural frequency of the swinging clack or flap as high as possible so that the valve closes quickly. Rapid-action means that no large reverse flow has time to build up before the valve closes. This is a radical solution to the problem of hammering.

The ideal check valve closes when the flow falls to zero but there is no check valve that does this when the flow reverses quickly. A certain reverse flow always occurs which causes hammering when the valve closes. In NAF-Check the disc is pivoted so that it swings as fast as possible - and the reverse flow is thereby minimised.

This is the principle incorporated in NAF-Check
A small angle of travel between fully open and close, a short distance between the axis of the shaft and centre of gravity of the disc and an oblique seating angle.

We draw the analogy between pendulum clock and a check valve to explain this principle.

Auxiliary Spring
The basic principles of NAF-Check design ensure the fastest natural closing speed so that the valve is uncomplicated and reliable and may be truely termed a free acting check valve. However, we recognise that there are services where rapid turning liquids will generate "water hammer" conditions even in a valve as quick closing as NAF-Check. To reduce this pressure surge to more acceptable limits, NAF-Check is designed to be fitted with auxiliary springs to accelerate the speed of closing. Thesesprings are easy to fit or remove and may be mounted to unsprung valves already in service if operating conditions warrant it. With NAF-Check, remember that auxiliary springs are just that, unlike other valves which rely solely on the spring for their basic action.

Even an unlikely event of a broken spring does not destroy the proper function of NAF-Check.

Installation & Maintenance
PN 25-40-100 Class
ANSI 150-300
Limitations
Certification
Sizing & Pressure Drop