Detecting apparatus with excess pressure protectors
BACKGROUND AND SUMMARY OF THE INVENTION 
     The purpose of this invention is to establish overpressure protection for devices used to measure various quantities and qualities of fluid systems. Some of these quantities and qualities are pressure, temperature, humidity, flow, etc. This device allows for overpressure protection without the necessity for relieving the total quantity of fluid which can be delivered through the fluid carrying elements (i.e., pipeline) leading to the sensing device. This is desirable, since if protection is established by relieving the elements &#39; full fluid carrying capacity, the relieving device becomes large and expensive. The invention is intended for protecting cable pressurization monitoring devices such as pressure transducers, flow transducers, pressure contactors, etc. These devices are normally subjected to pressures below 15 psig but occasionally may be subjected to higher pressure during component testing. The invention can be used for other purposes. 
     The protection device consists of an inlet flow controlling port followed by a pressure relieving poppet, and lastly a larger delivery port which connects to the sensing device. 
     The inlet flow control port is sized significantly smaller than the poppet relieving port. The sizing is determined by the pressure, P 1 , the delivery duct might carry to the sensing device and the pressure, P 2 , the sensing device is cable of withstanding. The size relationship is such that the inlet port is restrictive enough to limit flow with pressure differential P 1  - P 2  to the flow the poppet relieving port can carry with pressure differential P 2  -- atmospheric pressure or the normal discharge pressure of the environment at the exhaust of the relieving port. 
     In the case of protecting a cable pressure transducer (operating range of 0 - 10 psig and maximum pressure capability of 30 psig), an overpressure supply could be as high as 100 psig. This pressure could be carried through an inlet duct of 3/8 inch tubing. To protect the transducer from 30 psig overpressure would normally require a relieving device with a port size of 3/4 inch diameter. The selection of a proper ratio of inlet port diameter to poppet port effective diameter will allow for a maximum of 30 psig delivery through the delivery port. The actual size of the inlet port will be determined by the requirements of the sensing device. For example, if the sensing device requires large volumes of fluid, a very small inlet port will result in response time lags of the sensed quantity or quality. System requirements will dictate the necessary considerations as to response. 
     Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds. 
    
    
     BRIEF DESCRIPTION OF DRAWING 
     The drawing is a diagrammatic illustration of the preferred embodiment of the invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     A fluid-carrying element 10, which may be a pressurized cable, contains the fluid under substantial pressure which may be 10 psig during normal operation and which may be as high as 100 psig at times of over-pressure supply. Characteristics of this fluid pressure can be measured by measuring instruments comprising an instrument 12, and another instrument 14 connected with the element 10 through passages 16 and 18 and through intermediate housing 20 located at an intermediate region along the length of the passages 16 and 18. 
     The instrument 12 represents a detector of the nature which does not require that any fluid pass through it; for example, a pressure gauge. The instrument 14 is a dectector which requires that there be a flow of fluid passing through the detector; for example, a moisture detector. The rate of flow of fluid through the detector 10 is regulated by a valve 24 or other regulator for changing the rate of flow. Both of the detectors 12 and 14 are connected with the passage 18, which leads from the outlet side of the housing 20. 
     Within the housing 20, there is a chamber 26. Fluid flows from the passage 16 into the chamber 26 through an inlet port 28; and fluid flows from the chamber 26 to the passage 18 through an outlet port 30. 
     At the upper end of the chamber 26, there is a pressure relief valve 32 which includes a valve head 34 with packing 36 that contacts with a seat 38 when the relief valve is in closed position. 
     Above the valve seat 38, there is a port or bore 40 of larger diameter than the valve head 34, so that fluid can escape from the chamber 26 around the sides of the valve head 34 when the relief valve is in open position. The housing 20 is open at the upper end of the bore 40 for the escape of fluid into the ambient atmosphere. 
     The relief valve has a stem 42 extending downward through the chamber 26 and through an enlarged portion 44 of the chamber 26. In this enlarged portion 44, there is a nut 46 threaded on the lower end of the stem 42. This nut 46 provides a shoulder on the stem 42 for holding a spring 48 compressed against a fixed shoulder 50 located below the valve seat 34. If the shoulder is fixed on the valve stem, the pressure of the spring is constant; but by having the shoulder a threaded nut 46, the tension of the spring 48 can be adjusted as desired to change the pressure at which the relief valve will open. 
     The lower end of the stem 42 extends into a socket 52 in the upper end of a plug 54, which threads into an enlarged and threaded lower end of the chamber 26. 
     It is a feature of the invention that the inlet port 28 has a cross-section which is much less than the cross-section of the passage 16. This greatly limits the rate of flow of fluid from the passage 16 into the chamber 26. The outlet port 30 has a diameter substantially larger than the inlet port 28. There is no advantage in having the outlet port 30 restricted, and there is a disadvantage in that any unnecessary restriction beyond the chamber 26 causes delay in response of detector equipment to which fluid is supplied through the outlet port 30 and the passage 18. The restriction in the inlet port 28, on the upstream side of the chamber 26, is a necessary restriction for obtaining the advantages of this invention. 
     If an ordinary pressure relief valve were interposed between the passage 16 and 18, a very large and expensive relief valve would be necessary in order to carry away the fluid fast enough to prevent build-up of pressure in the passage 18 leading to the detectors which must be protected from excessive pressure. By having the housing 20 with the greatly restricted inlet port 28, this invention permits the use of a small housing and inexpensive relief valve, while at the same time providing full protection for the detectors against excessive over-pressure in the pressurized cable or other element in which pressure is contained. The greater the restriction, the smaller the relief valve can be. However, in systems where some of the detectors require a flow of fluid through them, the rate of fluid flow determines the size which must be used for the inlet port 28. In addition to the size relationships already pointed out in the introduction of this specification, the relationship must provide for the necessary total flow, and the inlet port will control the associated sizes of other ports so as to satisfy the relationships: 
     when P 1  = The maximum pressure possible in the element 10 that contains the process fluid. 
     when P 2  = The maximum pressure the chamber 26 can attain without resultant damage to instruments 12 and 14. 
     when P a  = The maximum pressure existing in the environment at the discharge of relief valve 32. 
     when F 28  = The capacity for fluid flow through port 28 at existing pressures P 1  and P 2 . 
     when F 32  = The capacity for fluid flow through relief valve 32 at pressures P 2  and P a . 
     then To protect instruments 12 and 14 F 32  ≧ F 28 . 
     Experience has shown that for the type of service for which this invention is intended, the ratio of the cross-section of the inlet port 28 to the equivalent cross-section of the relief valve 32 should be cross section 28=1, cross-section 32 ≧ 9. 
     Outlet port 30 and passage 18 should be sized to allow for good operation of instruments 12 and 14. In general, it will suffice to allow passage 18 to be large enough to handle any flow from fluid carrying element 10 to instruments 14 and 12, or vice versa (via breathing due to temperature or pressure change), without creating time lags that would prevent proper monitoring. 
     The housing 20 has connector portions 56 and 58 for securing it to the passages 16 and 18, respectively. It is important that the housing be connected with the passages 16 and 18 in such a way that the restricted passage 28 is at the upstream side of the housing 20. In order to prevent the housing 20 from being connected in the line backwards, the connections between the housing and the passages 16 and 18 may be different in construction, so that it is impossible to connect the housing in the line backwards. These incompatible connections can take various forms or arrows embossed on the device may be used to show the intended direction of flow. 
     In the construction illustrated in the drawing, the passage 16 extends into an annular socket 60 in the end face of the housing 20, and a flange 62 abuts against the housing and is clamped to the housing by a nut 64. 
     At the other end of the housing 20, a flange 66 on the passage 18 is clamped against an end face of the connector 58 by a nut 68, which screws over threads on the connector 58. These constructions for connecting the passages 16 and 18 with the housing 20 are merely representative of different structures for insuring the connection of the housing 20 with the inlet passage 28 at the upstream side of the housing. 
     The preferred embodiment of the invention has been illustrated and described, but changes and modifications can be made, and some features can be used in different combinations without departing from the invention as defined in the claims.