WO2003068634A1

WO2003068634A1 - Pressure relief device for silos

Info

Publication number: WO2003068634A1
Application number: PCT/GB2003/000606
Authority: WO
Inventors: William Liddle
Original assignee: Rmc Readymix Limited
Priority date: 2002-02-18
Filing date: 2003-02-11
Publication date: 2003-08-21
Also published as: GB2385407A; GB0203787D0; GB2385407B; AU2003245770A1

Abstract

The present invention relates to a pressure relief device for a silo comprising a body portion (20) defining an aperture closable by a hinged lid (10), the lid comprising a counterbalance (40), wherein a centre of gravity of the counterbalance (40), is aligned vertically with the pivot when the lid is in the closed position, such that the counterbalance (40) applies no or a negligible torque about the pivot when the lid is in a closed position.

Description

PRESSURE RELIEF DEVICE FOR SILOS

The present invention relates to pressure relief devices for use on silos containing particulate matter. In particular, the invention relates to an improved pressure relief device having improved opening and flow characteristics and for use on silos which are filled using a pressurised means such as pneumatic conveyance. Silos are a common means of storing particulate matter such as cement powder or grain. Commonly, the silos are filled from vehicles such as tankers by means of pneumatic conveyance, wherein the contents of the tanker are conveyed into the silo by means of a flow of pressurised gas, typically air. However, the silos themselves are not designed as pressure vessels and typically have a maximum working pressure limited to 6.9 KN/m/m (1 psig) . As such, there is a danger that when filling such silos by means of pneumatic conveyance or other similar means that the silo may become over-pressurised and subject to damage or explosion.

Pressure relief devices for such silos are known. The earliest and simplest form of pressure relief device was simply to leave one of the sheet metal covers of the silo axis manholes unsecured so that when an over-pressurisation condition occurred the cover flapped open. This rudimentary measure was imprecise and prone to leakage due to damp ingress through the cover. In addition, the presence of unsecured covers on the roof of the silo was a potential safety risk.

Other pressure relief devices are known which are of the spring-loaded type which use a lid which is movable relative to an aperture and is sprung using a helical spring to provide the sealing force on the lid against the valve seat around the aperture. However, such spring-loaded type pressure relief devices are known to have a tendency to stick shut due to build-up of the particulate matter within the coils of the internally mounted springs. In addition, the spring requires a progressively increasing force to open the aperture fully. For both reasons, on occasion, silos have been known to fail due to the inability of the pressure relief device to cope with the rapid increase in internal pressure which can occur during an over- pressurisation condition.

A further known type of pressure relief device is known as a rising lid deadweight device. In this type of device the aperture is closed by a lid which may be lifted off its seat when an over-pressurisation condition occurs. However, experiments have shown that such devices are incapable of responding quickly enough, and incapable of sustaining a sufficient flow capacity, in order to prevent the silo dangerously over-pressurising. In addition, the lid is prone to jamming open on actuation leading to a continuing leakage of the silo contents into the environment.

Traditionally, all pressure relief devices, other than the earliest flapping manhole type, have been designed using sharp edged orifice steady flow theory. The applicant has found by experiment that the use of such theories underestimates the required throat capacity of the device by a factor of two or more depending on the coefficients assumed in the calculations . It is an object of the present invention to provide a pressure relief device for a silo which substantially overcomes the problem of sticking and also provides a reliable and practical solution to the problem of over-pressurisation of silos. The present invention therefore provides a pressure relief device for a silo comprising a body portion defining an aperture closable by a hinged lid the lid comprising a counterbalance, a pressure relief device for a silo comprising a body portion defining an aperture closable by a hinged lid, the lid comprising a counterbalance, wherein a centre of gravity of the counterbalance is aligned vertically with the pivot when the lid is in the closed position, such that the counterbalance applies no or a negligible torque about the pivot when the lid is in a closed position. An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:-

Figure 1 is a cross-sectional view through a pressure relief device according to the present invention;

Figure 2 is a plan view of the pressure relief device of Figure 1;

Figure 3 is a side view of the pressure relief device of Figure 1;

Figure 4 is a schematic representation of the pressure relief device of Figure 1 in a fully open position; and

Figure 5 is a schematic representation of the pressure relief device of Figure 1 in a closed position.

As shown in Figures 1 to 3, the pressure relief device 1 of the present invention comprises a cylindrical body 20 mounted to a base plate 25 and a lid 10 hinged to the cylindrical body 20 by means of a hinge 30.

The cylindrical body 20 is formed from a section of steel tube and for convenience is preferably formed from a standard size of tubing. A lower end of the cylindrical body 20 is mounted to a base plate 25 by means of a fillet weld 24. The base plate 25 is also preferably formed from steel and is used to mount the pressure relief device 1 to the silo 26. The lower end of the cylindrical body 20 is open and in use communicates with the internal void space of the silo. The upper end of the cylindrical body is open and is closeable in use by the lid 10. An inwardly directed flange 21 is welded by means of a fillet weld 23 to the inner surface of the cylindrical body 20. A cylindrical body 20. A

through-hole 22 is provided in the inwardly directed flange 21.

The lid 10, as shown in Figure 2, comprises a steel plate which has a ' D' shape having a substantially circular portion and a rectangular portion towards the hinged edge of the lid. The 'D' shape of the lid 10 has the effect of altering the centre of gravity of the lid 10 to be nearer to the hinged edge than the centre of the pressure applied to the lid 10 in use which is centred on the circular portion of the lid 10 which is aligned with the cylindrical body 20. This has the effect of increasing the acceleration of opening of the lid 10. The circular portion has an external diameter which is greater than the external diameter of the cylindrical body 20. An annular gasket 11 is provided on a lower surface of the lid 10 and abuts in use against the upper rim of the cylindrical body 20 in order to provide a good seal therebetween. The gasket 11 may be formed from any suitable rubberised or elastomeric compound. The gasket 11 is preferably bonded to the lower surface of lid 10 by means of a glue or other adhesive. An annular drip rim 12 is mounted to the periphery of the lid 10 on a lower edge thereof to depend downwardly therefrom. The drip rim 12 serves to shield the annular gasket 11 and interface between the cylindrical body 20 and lid 10 from rain and water running off the upper surface of the lid 10 so as to prevent moisture ingress between the lid 10 and cylindrical body 20. The drip rim 12 is preferably welded to lid 10.

The lid 10 further comprises an inwardly directed flange 15 welded by means of a fillet weld 17 to the lower surface of the lid 10 at or near the centre thereof. The flange 15 is directed to be within the cylindrical body 20 when the pressure relief device is in the closed position as shown in Figure 1. A through-hole 16 is provided in the flange 15. In use, a chain or other shackle (not shown) is strung between through-holes 16 and 22. The use of the chain will be described further below. An upper surface of the lid 10 is provided with a threaded hole 13 at or near a centre thereof. The hole 13 may be used to secure additional ballast discs to the upper surface of the lid 10 if it is desired to increase the effective weight of the lid 10.

A section of tube 14 is mounted transversely by means of welding to the upper surface of lid 10. The tube 14 allows for the insertion of a shaft or rod to allow a user to apply leverage to the lid 10 to aid the manual opening of the pressure relief device 1 during installation or maintenance.

The lid 10 is hinged to the cylindrical body 20 by means of a hinge 30. Hinge 30 comprises two downwardly depending flanges 31 welded to the underside of the rectangular portion of the lid 10 and two outwardly extending flanges 32 which are mounted to the cylindrical body 20 by means of fillet welds 42 . Both the downwardly depending flanges 31 and outwardly depending flanges 32 are rotatably mounted on a hinge pin 33 and are rotatable thereabout. End plates 34 are provided on hinge pin 33 so as to prevent hinge pin 33 being drawn out of the holes in the flanges 31 and 32.

In operation of the pressure relief device 1, an over-pressurisation of the silo will create an internal pressure in the silo which will create a force acting upwardly on the lower surface of lid 10. When the pressure in the silo reaches a critical value (known as the 'unstick' pressure) , the force applied to the lower surface of the lid 10 is sufficient to unstick the lid 10 from the cylindrical body 20 and rotate it in an anti-clockwise direction when viewed as in Figure 1. The unsticking of the lid 10 from the cylindrical body 20 allows the pressurised gas in the silo to rapidly vent to atmosphere.

The chain strung between holes 16 and 22 limits the angle ( ) of opening of the lid 10 to a predetermined value so as to avoid damage to the hinge 30 and to ensure that the lid 10 is capable of closing once the over-pressurisation has diminished. Preferably, the chain is 250 mm in length ensuring that the chain cannot become trapped between the lid 10 and the cylindrical body 20 as the lid 10 closes. This enables the designed maximum opening angle α to be achieved without risk of damage to the elastomeric seal, the sealing face of the cylindrical body 20 or to the chain itself. Oscillation and pressure pulsation problems can occur with pressure relief devices having a hinged lid since once the lid is open the airflow through the device decays as the pressure in the silo diminishes . As a result the force holding the lid in the open position also diminishes leading to the lid swinging shut at which point the albeit diminished airflow is sufficient to re-open the lid. This oscillation of the lid can be violent which leads to the danger of mechanical damage to the pressure relief device and also to the setting up of pressure pulsations in the silo itself which can lead to failure .

According to the present invention, a counterbalance in the form of a weight 40 is mounted by means of a fillet weld 41 to the upper surface of lid 10 and positioned such that the centre of gravity of the counterbalance 40 lies vertically above the centre of rotation of hinge pin 33 as shown in Figure 1 so that the counterbalance 40 exerts no torque about the pivot in the closed position of lid 10. In addition, since the weight of the counterbalance 40 is directed vertically through the pivot point and is supported directly by the hinge pin 33 it has little or no effect on the static sealing load of the lid 10.

Preferably, the maximum angle . of lid inclination is chosen to be in the range of 50 to 60 degrees. More preferably, the maximum angle is 55 degrees to generate a sufficient moment to initiate lid closure and to overcome windage effects on the open lid.

As the lid 10 starts to rotate in an anti- clockwise direction as viewed in Figure 1, the centre of gravity of the counterbalance 40 moves out of alignment with the axis of rotation of hinge pin 33. Consequently, the counterbalance 40 applies a torque to the lid 10 tending to urge the lid 10 to open. As such, this assists the opening of the lid 10 and increases the acceleration of the lid 10. The torque applied increases progressively as the opening angle . increases and is a maximum at the point of maximum lid inclination. At this point, it is preferable that the torque applied by the counterbalance 40 cancels out the majority of the torque generated in the reverse direction by the mass of the lid 10. As a result, the force required to hold the lid 10 fully open is reduced with the effect that the necessary air flow through the cylindrical body 20 which is required to maintain the lid 10 in the fully open condition is similarly reduced allowing a longer period of unrestricted air flow through the pressure relief device 1 so that the lid 10 remains fully open for longer.

In addition, since the lid 10 does not start to close until the air flow through the pressure relief device 1 is commensurately lower the degree of oscillation of the lid 10 and its associated pressure pulsation within the silo is significantly reduced. The mass and location of the counterbalance 40 will vary depending on the mass of the lid 10, the chosen maximum opening angle . and the chosen unsticking pressure. The following example is therefore exemplary in nature and is based on adapting the a pressure relief device which has an effective area of 61 300 mm² (95 in²) . With such a device, it has been found by experiment by the applicant that a device having an effective area of 61 300 mm² (95 in²) when used with a tanker pressurised to 2 barg (2 atmospheres above ambient) and a desired unstick pressure of 4.83 KN/m/m (0.7 psig) requires a lid having a mass of 29.94 kg (66 lbs) to avoid over-pressurisations of more than 6.9 KN/m/m (1 psig). The unstick pressure must be sufficiently above the normal working pressures of the silo, 1.03-1.38 KN/m² (0.15-0.20 psig) , so as to prevent unwanted openings of the device in non-overpressurisation conditions and also sufficiently below the pressure rating of the silo, 6.90 KN/m² (1.0 psig), to allow for an adequate safety margin. In addition, a minimum acceleration of the lid may be calculated which must be met in order to meet the pressurisation requirements. The minimum acceleration may be calculated by assuming that the maximum permitted pressurisation of 6.9 KN/m/m (1 psig) is acting on the lid.

Since no venting of air takes place until the lid 10 is unstuck and since at the point of unsticking the force applied to the lid 10 must equal the weight of the lid 10 it can be assumed that the maximum residual force available to accelerate the lid to fully open is the total available force less the weight of the lid. (In practice the residual force will increase from zero at the point of unsticking to the maximum residual force at 6.9 KN/m/m (1 psig) . However, this occurs so rapidly that for these calculations it has been assumed that 6.9 KN/m/m (1 psig) acts throughout) . As mentioned above the weight of the lid 10 is 29.94 kg (66 lbs). Thus, the residual force equals:

Total available force - Weight of lid

(6.9 KN/m2 * 0.0613 m2 ) - (29.94 kg * 9.81)

0.423 KN - 0.294 KN

0.129 KN (29 lb force)

Thus, the minimum acceleration equals: .

0.129 KN / 29.94 kg = 4.31 m/s/s

(14.15 ft/s/s)

Consequently, the equivalent pressure relief device according to the present invention having an effective area of 61 300 mm² (95 in²) requires a lid acceleration no less than 4.31 m/s/s to prevent the pressure in the silo exceeding 6.90 KN/m².

Example

The addition of the counterbalance 40 increases the inertia of the lid which requires a reduction in the mass of the lid.

Normal maximum working pressure of silo = 6.9 KN/m/m (1 psig) Silo filled by tanker pressurised to 2 barg Effective area of lid exposed to silo = 61 300 mm² (95 in²)

Mass of lid= 25.66 kg (56.57 lbs)

Consequently, unstick pressure = 4.12 KN/m/m (0.6 psig)

Mass of counterbalance= 6.38 kg (14.07 lb) u = 122.5 mm p = 160 mm

Maximum lid inclination, , is chosen as 55 degrees.

Referring to Figure 4, at an α of 55 degrees the offset of the lid centre of gravity, x, from the pivot is 28.28 mm (1.113").

Similarly, the centre of gravity of the counterbalance 40 is offset by y = 100.35 mm (3.95")

Moment applied by counterbalance = 640 kgmm Moment applied by lid = 726 kgmm

Therefore, % counterbalance = 640/726 = 88%

The residual force acting on the lid and counterbalance equals:

0.423 KN - 0.252 KN = 0.171 KN (38.4 lb force)

Referring to Figure 5, the inertia of the lid 10 acts vertically down through the centre of gravity of the lid 10 and equals the weight of the lid:

25.66 kg

The inertia of the counterbalance 40 about the centre of gravity of the lid equals:

6.38 kg * (u/p) 6.38 kg * (122.5 mm /160 mm)

4.88 kg

Total inertia equals:

4.88 kg + 25.66 kg = 30.54 kg

Acceleration equals:

0.171 KN / 30.54 kg = 5.59 m/s/s

Compared to acceleration of conventional lid:

5.59 / 4.31 = 1.29 or an increase of 29%

Consequently, the counterbalanced lid of the present invention opens 29% faster than the non- counterbalanced pressure relief device lid.

The counterbalanced lid has the advantage that the faster opening of the lid will enable the lid to reach its fully open position more quickly enabling the device to better handle the initial maximum pressure within the silo and the flowrate transient. In addition, the counterbalancing (88%) in the fully open position increases the lid dwell time in the fully open position which increases the overall venting capacity of the device and shortens the venting cycle. oscillations and pressure pulsation are also significantly reduced if not eliminated.

Advantageously, the device may be manufactured wholly from standard plate and tube sizes from steel. The device may also be easily retro-fitted to existing silos since the external shape and dimensions of the cylindrical body have not been changed.

Claims

Claims :

1. A pressure relief device for a silo comprising a body portion defining an aperture closable by a hinged lid, the lid comprising a counterbalance, wherein a centre of gravity of the counterbalance is aligned vertically with the pivot when the lid is in the closed position, such that the counterbalance applies no or a negligible torque about the pivot when the lid is in a closed position.

2. The pressure relief device of claim 1 wherein the lid is hinged to the body portion about a pivot and on opening of the lid, the torque applied by the counterbalance about the pivot opposes the torque applied by the lid about the pivot.

3. The pressure relief device of claim 1 or claim 2 wherein the magnitude of the torque applied by the counterbalance about the pivot is between 80 and 95% of the magnitude of the torque applied by the lid about the pivot when the lid is in a fully open position.

4. The pressure relief device of claim 3 wherein the magnitude of the torque applied by the counterbalance about the pivot is approximately 88% of the magnitude of the torque applied by the lid about the pivot when the lid is in a fully open position.

5. The pressure relief device of claim 3 or claim 4 wherein in the fully open position the lid is angled relative to the body portion at between 50 and 60 degrees.

6. The pressure relief device of claim 5 wherein in the fully open position the lid is angled relative to the body portion at approximately 55 degrees.

7. The pressure relief device of any preceding claim wherein the lid and body portion are coupled together to limit the maximum angle of inclination of the lid relative to the body portion.

8. The pressure relief device of claim 7 wherein the coupling is by means of a chain or similar.

9. A silo comprising a pressure relief device as claimed in any preceding claim.

10. A pressure relief device substantially as hereinbefore described with reference to and as shown in the accompanying drawings .