Various forms of diaphragm valves have been proposed and include those disclosed in GB-A-808074 and GB-A-926129 and include a body, for example, a cast iron body, including a tubular conduit providing a substantially straight through flow path in the open position of the valve, a flexible diaphragm moveable within the housing, means, which may be, for example, in the form of a so-called bonnet and an associated pressure plate driven, by a hand wheel or the like, which urges the diaphragm against a mid- portion of the tubular conduit to close the valve. The pressure plate, also known as a compressor, is engaged against the upper or outer surface of the diaphragm and is essentially formed integral therewith. Part of the upper and lower surfaces of the flexible diaphragm, at its periphery, are curved in planes parallel to the direction of movement of the diaphragm.

Such valves operate very satisfactorily and, because of the straight through flow path, can drain down easily when not in use. However, there is a tendency for the valve not to seal fully and a fairly considerable effort is required to operate the compressor to move the valve to the closed position.

It is an object of the present invention to provide an improved valve of this general type.

It is now proposed, according to one aspect of the invention, to provide a method of manufacturing a diaphragm valve, comprising the steps of providing a diaphragm of flexible material, said diaphragm having an upper surface and a lower surface, providing a valve body having a through conduit, extending between an inlet and an outlet, experimentally determining the shape of the lower surface of the diaphragm when in its fully lowered "valve closed"

position and forming a seat portion said seat portion having a shape accurately to match said determined shape of the lower surface of the diaphragm in its fully lowered "valve closed" position.

Previously the designing of the valve has, in essence, been the reverse of this. Thus, the seat shape is the starting point and the diaphragm is designed to fit the seat. Although reasonably close attempts can be made to give a thorough seal, it is not in fact practical to do so.

According to the invention, however, one can experimentally determine exactly the shape of the diaphragm in the "valve closed" position and then design the valve seat accurately to conform to this.

Preferably the method of the invention further comprises the step of providing a shaped compressor to act on the upper surface of the diaphragm, experimentally determining the shape of the upper surface of the diaphragm, when in its fully lowered "valve closed" position and forming the lower surface of the compressor of a shape accurately to match said determined shape of the upper surface of the diaphragm in its fully lowered "valve closed" position.

In prior constructions the shape of the lower surface of the compressor normally is such that the upper surface of the diaphragm fully engages the compressor well before the valve reaches the "valve closed" position.

Using both of the preferred features of the invention, therefore, one can achieve a valve which is substantially leak proof even at significantly higher pressures than have been hitherto possible and the act of closing the valve requires less effort than has hitherto been possible.

A further advantage is that it is found that with the diaphragm in the "valve open" position, the through flow of fluid can be significantly increased for a given pressure

drop from the inlet to the outlet of the tubular conduit.

The invention also provides a diaphragm valve produced by the method of the present invention.

In order that the present invention may more readily be understood, the following description is given, merely by way of example, reference being made to the accompanying drawing in which the sole figure is a cross- section through one embodiment of diaphragm valve, the left hand portion shown in the drawing indicating a valve according to the prior art, and the right hand portion in the drawing showing one embodiment of diaphragm valve according to the invention.

The valve illustrated in the drawing, on the left hand side, consists of a valve body 10 having a through passage 12 provided with an inlet pipe flange 14 and an outlet pipe flange 16. A diaphragm 18 is mounted on an upper surface of the body 10 and is held in position by a bonnet 20.

The diaphragm has, associated with its upper surface 22 an upwardly extended moulded-in bolt 24.

Threaded on to this bolt is a shaped compressor 26 having a curved lower surface 28 and an upwardly extending tubular spigot 30, the inner surface of which is provided with a female thread. Threaded into this is the male thread of an actuating spindle 32, which is secured to a hand-wheel 34 by means of a bolt 36. An indicator rod 38 extends upwardly through the hand-wheel 34 to indicate the position of the compressor 26 and thus of the diaphragm 18.

As can be seen the diaphragm has two sinuous folds, one on each side of the bolt 24, joined by two transverse folds. The diaphragm is formed of a suitable rubbery material with a fabric-like reinforcement which, as is generally conventional, is slightly nearer the top surface of the diaphragm than the bottom.

When the hand-wheel 34 is rotated, the spigot 32

is screwed downwardly thereby pushing the compressor 26 downwardly and with it the bolt 24 which in turn pushes the diaphragm downwardly into engagement with a valve seat surface 13 of the conduit 12.

The present invention is significantly improved over the structure illustrated on the left, insofar as the shape of the valve seat indicated by 15 is determined, by experiment, accurately to conform to the shape of the lower surface 23 of the diaphragm 18 when the diaphragm is in the fully closed position. It will be seen that the surface 15 is lower than the surface 13. This is because it has now been appreciated, according to the invention, that because the reinforcement of the diaphragm is above the centre of thickness of the diaphragm, this creates a longer neutral axis and the diaphragm can move downwardly further than was previously perceived and the valve seat, which conforms accurately to the lower surface of the diaphragm is thus lower. In this way the diaphragm will fully seal against the valve seat portion 15 around the full length of that valve seat portion and the possibility of any leakage will be significantly diminished. Because the seat 15 is lower, the fluid can flow more readily through the valve in the open position.

It will also be noted that the lower surface 29 of the compressor 26 is significantly different from the lower surface 28 of the prior art. With the structure of the invention, this lower surface shape is also determined by the shape of the upper surface 22 of the diaphragm when the valve is in the fully closed position. In this way, the upper surface of the diaphragm will be engaged fully against the lower surface 29 of the compressor 26 substantially at the point of closure thereby diminishing the compression of the diaphragm which is necessary and also greatly diminishing the effort required to operate the compressor, reducing, in this instance, the torque which needs to be

applied to the hand-wheel or reducing the force required on any other actuating arrangement.

The structure according to the invention also provides, which in itself is to the applicants' knowledge novel, the concept of the indicator rod 38 being itself threaded into the bore 40 in which the diaphragm bolt 24 is itself threaded. Thus the same threaded bore is used for both purposes. The positioning of the indicator rod 38 into the compressor 26 provides a clear indication of the position of the diaphragm.