GB2167842A

GB2167842A - Safety valve

Info

Publication number: GB2167842A
Application number: GB08430520A
Authority: GB
Inventors: Robert Meadows
Original assignee: Crosweller & Co Ltd W; Kohler Mira Ltd
Current assignee: Crosweller & Co Ltd W; Kohler Mira Ltd
Priority date: 1984-12-04
Filing date: 1984-12-04
Publication date: 1986-06-04
Also published as: GB8430520D0; GB2167842B

Abstract

A safety valve 2 operable to release automatically a large increase in fluid pressure beyond the normal fluid working pressure in a hollow casing 3 of an instantaneous water heater 1. The safety valve 1 comprises a silicone rubber plug 10 located in a through bore 9 of a tubular element 8 detachably mounted in an opening 12 in the casing wall. The plug 10 is an interference fit in the bore 9 to seal the bore under the normal fluid working pressure. The bore 9 has an orifice 18 of reduced diameter and the plug 10 is ejected from the bore by an increase in fluid pressure within the casing sufficient to compress and extrude the plug 10 through the orifice 18. The pressure increase required to eject the plug 10 is controlled by the physical characteristics of the silicone rubber and the relative dimensions of the plug 10 and orifice 18 and may be set for any given application as desired. <IMAGE>

Description

SPECIFICATION Safety valve This invention relates to safety valves and in particular to a safety valve operable to release automatically an excessive increase in fluid pressure beyond a normal working pressure.

The invention has special application to water heaters and in particular, though not exclusively, to so-called instantaneous electric water heaters.

Instantaneous electric water heaters provide a source of hot water on demand and are commonly provided in domestic ablutionary installations, especially for showering, either when a supply of hot water from a conventional storage cylinder is not readily available or when the demand for hot water is irregular.

The known instantaneous electric water heaters generally comprise a hollow casing of fixed volume having an inlet for connection to a supply of cold water, an electric heating element for heating the water and an outlet for connection to an ablutionary appliance. In use, cold water flows continuously through the casing and is heated to the required temperature by the heating element so that only that water which flows through the casing for immediate use is heated. In this way the amount of electrical energy used is often less than that required to heat and maintain at a raised temperature until required a relatively large volume of water contained in a storage cylinder thereby enabling a saving in energy costs to be made.

In order to obtain the necessary heat transfer to heat rapidly the water flowing through the casing to the required temperature, a heating element of considerable heating capacity is required, generally about 7 kW. As a result, if the heating element is switched on with the outlet either mechanically closed or blocked, for example frozen, the water or ice in the casing will be rapidly transformed into steam with consequential large increase in volume and hence pressure beyond the normal working pressure giving rise to a significant risk of the casing exploding. Accordingly, in order to safeguard against the risk of explosion under these conditions there is a need to provide a safety valve which will operate to release automatically any excessive increase in fluid pressure beyond the normal working pressure and which the casing is designed to withstand safely.

The safety valve provided must be reliable in operation and must remain operable even after a prolonged period of time, for example the water heater may be installed and used for several years before operation of the safety valve is required. Furthermore, the safety valve must comply with Water Authority Regulations. In particular, the valve must not contain any materials which may contaminate the water and should preferably be inert to the water under all operating conditions.

Known valves which satisfy the foregoing requirements are of complex construction and are expensive both to manufacture and install.

It is an object of the present invention to provide a safety valve which is of simple construction and is cheaper both to manufacture and install than the known safety valves.

It is a further object of this invention to provide a safety valve which can be easily reset after use.

According to the present invention, in a fluid heater comprising a hollow casing of fixed volume having an inlet for supply of fluid to be heated, means for heating the fluid and an outlet for delivery of heated fluid, there is provided a safety valve comprising an elastomeric plug located in a through bore in a wall of the casing, the plug being an interference fit in the bore such that, in use of the heater, the plug seals the bore and remains in the bore under the normal working pressure of fluid in the casing and, in the event there is a large increase in fluid pressure within the casing beyond the normal working pressure, the plug is forced out of the bore by the increased fluid pressure to release automatically the increased fluid pressure.

The invented safety valve consisting of a displaceable elastomeric plug located in a through bore is of extremely simple construction which can be easily manufactured to provide the necessary interference fit of the plug in the bore, is easy to install and can be reset after use simply by re-inserting the plug into the bore.

Preferably the plug is of spherical shape and the bore has a part-spherical seating against which the plug abuts. Advantageously the bore is of stepped construction having first and second cylindrical parts connected by the seating. The first part, which in use opens into the casing, is preferably of slightly smaller diameter than the uncompressed diameter of the plug to compress lightly the plug and ensure a fluid-tight seal is obtained under normal working pressures. The second part, which in use opens to atmosphere, defines an orifice of reduced diameter relative to the first part to provide the interference fit of the plug in the bore sufficient to retain the plug in the bore under normal working pressures.

As will now be understood, in order to eject the plug from the bore, the plug has to be further compressed by an increase in the fluid pressure in the casing beyond the normal working pressure sufficient to extrude the plug through the orifice. Accordingly, by appropriate seiection of the elastomeric material forming the plug and the relative diameters of the plug and orifice, the fluid pressure required to eject the plug can be controlled. Preferably the fluid-pressure required to eject the plug is 25% greater than the normal working pres sure. We have found that this is sufficient to prevent ejection of the plug due to minor fluctuations in the working pressure whilst ensuring that a large increase in fluid pressure within the casing is quickly released by ejection of the plug.

The plug consists of an elastomeric material of which the properties comply with the Water Authority Regulations relating to potable water installations and which retains its physical characteristics over a wide operating temperature range for a prolonged period of time. A suitable elastomeric material is silicone rubber having a hardness of 60 IRHD or less.

We have found that, by using silicone rubber of 50 IRHD hardness, a plug having a long working life and consistent operating performance is obtained.

The bore may be formed in the wall of the casing itself, but more preferably the bore is provided by a tubular element which is detachably mounted in an opening provided in the wall of the casing. In this way not only is assembly and fitment of the safety valve facilitated but, after operation of the safety valve to release a large increase in fluid pressure beyond the normal working pressure, the valve can be easily re-set by removing the element, inserting the plug into the end of the first bore part to abut against the seating and re-fitting the element in the opening in the enclosure wall.

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawing wherein: FIGURE 1 is a cross-section through a water heater incorporating a safety valve according to the present invention; FIGURE 2 is a section, to an enlarged scale, on the line X-X of Figure 1; and FIGURE 3 is a cross-section, to an enlarged scale, through part of the valve shown in Figure 1.

Referring to the drawing, an instantaneous electric water heater 1 is shown incorporating a safety valve 2 according to the present invention. In known manner, the water heater comprises an elongate hollow casing 3 having an inlet 4 for connection to a supply of cold water, a plurality of elongate heating elements 5 extending lengthwise of the casing 3 for heating the water and an outlet 6 for delivery of heated water, for example to a shower rose or tap (not shown). The heating elements are of the electrical resistance type having a respective terminal 7 at each end for connection to a power source. In use water is circulated continuously through the casing 3 and heated rapidly to the required temperature by the heating elements 5 which have a large heating capacity, for example 7kW.

Additionally, the water heater includes suitable controls (not shown) for regulating the flow of water through the casing 3, selecting the temperature of the heated water produced and monitoring and maintaining the selected temperature. Such controls are well known to those skilled in the art and will not be further described herein as they form no part of the present invention.

The safety valve 2 is provided to release automatically a large increase in fluid pressure within the casing 3 beyond the normal fluid working pressure and comprises a tubular element 8 providing a through bore 9 in which a spherical plug 10 of silicone rubbber of 50 IRHD hardness is located. Sjlicone rubber is selected for the plug 10 because of its inertness to water as required by the Water Authority Regulations and its excellent retention of physical characteristics e.g. hardness, resilience, elasticity etc., over the operating temperature range for a long period of time.

The element 8 has a cylindrical body portion 11 provided with an external screw thread by means of which the element 8 is detachably mounted in an opening 12 in the wall of the casing 3. The element 8 further has a hexagonal head 13 to facilitate fitment and tightening by means of a suitable spanner and a fibre washer 14 positioned under the head 13 provides a fluid-tight seal between the element 8 and the casing 3.

The through bore 9 is of stepped construction having cylindrical first and second parts 15 and 16 respectively connected by a partspherical seating 17 against which the plug 10 abuts in the assembled condition of the valve.

The first bore part 15 opens into the casing 3 and has a diameter slightly less than the uncompressed diameter of the plug 10 to compress lightly the plug 10 on assembly and ensure a fluid-tight seal is obtained between the plug 10 and the seating 17. The second bore part 16 defines an orifice 18 opening to atmosphere and has a diameter less than the first bore part 15 to provide an interference fit of the plug 10 in the bore 9 sufficient to retain the plug 10 in the bore 9 under normal fluid working pressures in the casing 3.

The relative diameters of the plug 10 and the orifice 18 are so chosen together with the physical characteristics of the silicone rubber forming the plug, for'example hardness, resilience, elasticity etc, that a 25% increase in the fluid pressure within the casing 3 beyond the normal fluid working pressure is required to compress further the plug 10 and extrude the plug 10 through the orifice 18 thereby releasing the fluid pressure within the casing by allowing the fluid to escape through the bore 9.

This pressure difference avoids erroneous operation of the valve due to minor fluctuations in the normal fluid working pressure whilst ensuring a fast response to a sudden large increase in fluid pressure within the casing 3.

After operation, the safety valve may be reset by removing the element 8 from the opening 82 in the casing wall, re-inserting the plug into the first bore part 15 and re-mounting the element in the opening. We have found that the pressure at which the plug 10 is ejected from the bore 9 remains substantially constant if the plug is inserted wet.

As will now be understood the present invention provides a safety valve which is of simple construction thereby facilitating manufacture and installation, which is reliable in operation even after a prolonged period of time and which can be re-set after use.

When the safety valve is fitted to a water heater as above-described, it is envisaged that the casing 3 would be mounted in a housing to avoid any risk of a user being injured by the ejection of the plug 10 from the orifice 18 or scalded by the hot fluid escaping through the bore 9. Additionally the housing would include an outlet to direct escaping fluid to a waste.

By use of the safety valve according to the present invention in a water heater, especially of the instantaneous type, risk of explosion of the casing due to the heating element(s) being switched on when the outlet is either mechanically closed or blocked, for example frozen, is eliminated in a simple and effective manner.

Although the invented safety valve has been described with particular reference to its application in an instantaneous water heater it will be understood that the safety valve may be used in any type of fluid heater having a hollow casing in which a fluid is heated.

It will be appreciated that the invention is not limited to the construction of safety valve abovedescribed which may be modified in a number of ways, for example the plug may be formed from elastomeric materials other than silicone rubber, the material selected depending on the fluid to be heated for any given application.

The relative dimensions of the plug and orifice and the physical characteristics of the plug material may be selected to control the pressure at which the plug will be displaced as desired.

Finally, the through bore in which the plug is located may be provided by a tubular element as described or it may be formed directly in the casing wall. The use of a detachable element is preferred however as facilitating resetting of the valve after use.

Claims

1. In a fluid heater comprising a hollow casing of fixed volume having an inlet for supply of fluid to be heated, means for heating the fluid and an outlet for delivery of heated fluid, there is provided a safety valve comprising an elastomeric plug located in a through bore in a wall of the casing, the plug being an interference fit in the bore such that, in use of the heater, the plug seals the bore and remains in the bore under the normal working pressure of fluid in the casing and, in the event there is a large increase in fluid pressure within the casing beyond the normal working pressure, the plug is forced out of the bore by the increased fluid pressure to release automatically the increased fluid pressure.

2. In a fluid heater, a safety valve according to claim 1 wherein the plug is spherical and the bore has a part-spherical seating against which the plug abuts.

3. In a fluid heater, a safety valve according to claim 2 wherein the bore is of stepped construction having first and second parts connected by the seating.

4. In a fluid heater, a safety valve according to claim 3 wherein the first bore part opens into the casing and has a diameter slightly less than the uncompressed diameter of the plug.

5. In a fluid heater, a safety valve according to claim 3 or claim 4 wherein the second bore part opens to atmosphere and defines an orifice of reduced diameter relative to the first bore part.

6. In a fluid heater, a safety valve according to claim 5 wherein the physical characteristics of the elastomeric material of the plug and the relative dimensions of the plug and orifice are so chosen that an increase in pressure of 25% above the normal working pressure in the casing is required to eject the plug from the bore.

7. In a fluid heater, a safety valve according to any one of the preceding claims wherein the plug is formed from silicone rubber.

8. In a fluid heater, a safety valve according to claim 7 wherein the silicone rubber has a hardness of 60 IRHD or less.

9. In a fluid heater, a safety valve according to claim 8 wherein the silicone rubber has a hardness of 50 IRHD.

10. In a fluid heater, a safety valve according to any one of the preceding claims wherein the through bore is provided by a tubular element detachabiy mounted in the casing wall.

11. In a fluid heater comprising a hollow casing of fixed volume having an inlet for supply of fluid to be heated, means for heating the fluid and an outlet for delivery of heated fluid, there is provided a safety valve substantially as hereinbefore described with reference to the accompanying drawing.