WO2007114703A1

WO2007114703A1 - An arrangement in a fire water system

Info

Publication number: WO2007114703A1
Application number: PCT/NO2006/000121
Authority: WO
Inventors: Nils T. Ottestad; Finn WICHSTRØM
Original assignee: Aker Kvaerner Engineering & Technology As; Ottestad Breathing System As
Priority date: 2006-04-03
Filing date: 2006-04-03
Publication date: 2007-10-11

Abstract

An arrangement in an offshore fire water system, particularly for an offshore platform, the system including one or more fire water pumps connected to a number of deluge valves (9) by way of a pipe system (8), to direct water to respective fire sections, and the arrangement including at least one water-filled buffer tank (1) containing an upper gas-filled volume connected to a gas supply device. The gas-filled volume is connected to the outlet of a high capacity gas supply valve (2) having a preset delivery pressure, and the buffer tank (1) includes an outlet (3) arranged low, the outlet being connected to the pipe system (8) by way of a control valve (7) which, cooperating with a pilot valve (4), is arranged to close very quickly if the water level in the buffer tank (1) reaches a lower limit.

Description

Arrangement in a fire water system

The invention relates to an arrangement in a fire water system, especially on an offshore platform, the system including one or more fire water pumps connected to a number of deluge valves by way of a pipe system, to direct water to respective fire sections, and the arrangement including at least one water-filled buffer tank containing an upper gas-filled space connected to a gas supply device. A fire water system including an arrangement of the type mentioned above is known from NO patent No. 314 022 (corresponding to WO 02/36208). In this system compressed air is used as driving force to press the water from one or more buffer tanks through normally dry water stretches. The purpose is to have fire water transported to the point of warning before the fire water pumps have started and can provide for the further delivery of fire water.

Experience from gas fires on offshore platforms has resulted in that lately there has been focused on the need to be able to get water quickly to a scene of fire or to an area where a gas leakage has been presaged. In such situations one wants a quick influx of water to cool down the area, thereby preventing ignition of the gas or counteracting an escalation of a possible fire attempt.

In a traditional offshore fire water system one will not be able to achieve a stable supply of fire water to a possible scene of fire before the fire water pumps have started working. The start-up time for large fire water pumps typically will be 20 seconds or more. The piping arrangement in the fire water system is normally kept under pressure by smaller feed pumps, so-called jockey pumps. The deluge valves, which are to open for water supply to the area in question, will normally open quickly after a warning has been registered. This means that the consumption of water within a short time will exceed what the feed pumps manage to deliver. The consequence will be that column separation occurs in high-lying parts of the pipe system. When the fire water pumps start working, water will quickly be pumped up to these areas of the system, and as the vacuum pockets in the pipes collapse there may occur such strong pressure pulses that the pipe system bursts. In order to counteract this problem one may change start-up procedures or install special devices preventing that such vacuum pockets may arise. In general, this type of measures will additionally delay the fire water supply to a fire section.

The object of the invention is to provide a fire water system having an arrangement causing that one may have established a stable supply of fire water to a fire section approximately immediately after a warning of a gas leakage or a fire has been given, without the risk that pressure surges occur which may put the fire water system out of action.

The above-mentioned object is achieved with an arrangement of the introductorily stated type which, according to the invention, is characterized in that the gas-filled volume of the buffer tank is connected to the outlet of a high capacity gas supply valve which is connected to the gas supply device and has a pre-set delivery pressure, and that the buffer tank has a low-placed outlet which is connected to the pipe system by way of a control valve which, cooperating with a pilot valve, is arranged to close very quickly if the water level in the buffer tank reaches a lower limit. The arrangement according to the invention is based on the buffer tank being connected to that part of the pipe system which in a normal situation is kept pressurized to for example 12 bar by the jockey pumps. When the deluge valves of the system are activated, the consumption of water after a short time will exceed the capacity of the jockey pumps, so that the pressure in the pipe system starts falling. As the pressure above the water surface at the top of the buffer tank has reached down to the preset supply pressure for the gas supply valves of the system, these will start delivery of such great quantities of air that the preset pressure is maintained. Thereby also stable pressure conditions are maintained in the pipe system, and the fire section in question gets a defined fire water supply that can be maintained as long as there is water in the buffer tanks. The total capacity of the buffer tanks is evaluated from the start-up time of the fire pumps and the water demand of the respective fire sections at relevant fire scenarios. The necessary total volume of the buffer tanks typically may be in the region 5-20 m , depending on the individual installation. There will always be a certain risk that the fire water pumps do not start up within the prescribed time frames. This should be taken into consideration when the capacity of the buffer tanks is decided.

If an error occurs, the start-up in any case may be delayed to the extent that the buffer tanks are emptied of water before the fire water pumps start working. The invention is, starting from the function of the control valves of the system, arranged to minimize the problems that may occur as a consequence of such a situation. If there is much air in the pipe system, the fire water pumps must be started very carefully if one is to avoid pressure surges in the pipe system. In this situation it will be necessary to delay the start-up additionally in order to avoid problems.

The control valve has a dual function. One function is to shut off the connection between the buffer tank and the pipe system immediately before all the water is pressed out of the buffer tank. The other function is to utilize the elastic gas volume in the buffer tank to smooth out pressure pulses that might occur in the pipe system when the fire water pumps start up. The control valve therefore is arranged such that it opens immediately from a possibly closed position as the pumps start up and create a sudden pressure increase in the system. The invention may be combined with other technical solutions in order to reduce the risk for pressure surges in connection with fire water pumps not coming into operation within the normal start-up time. A relevant solution may be that a pressure surge compensator is installed at the highest point of the pipe system (cf. NO patent No. 308 183). The pressure surge compensator in a controlled manner establishes a pressure surge-absorbing gas buffer in high-lying parts of the system, and is in use on platforms in the North Sea with the purpose of minimizing the risk for pressure surges. The invention will be further described in connection with an exemplary embodiment with reference to the drawings, wherein Fig. 1 shows the main components of a system having an arrangement according to the invention;

Fig. 2 shows a sectional view of a preferred embodiment of the control valve; and

Figs. 3a and 3b show sectional views of the pressure-sensing control valve in two different modes.

Fig. 1 shows a partly sectioned assembly of the main components in an arrangement according to the invention. The arrangement includes a water-filled buffer tank 1 which in a normal situation contains a gas volume in the upper part constituting roughly 5 % of the total volume of the buffer tank. The gas volume is kept within specific limits by a non-illustrated float device which dumps gas to the surrounding atmosphere if the water level is too low, and which correspondingly supplies gas if the water level is too high. One or more high-capacity gas supply valves in the form of gas regulators 2 are connected to the buffer tank with their outlets in said gas volume. The gas regulator is supplied with high pressure gas (air) via the illustrated inlet 10 which is connected to a gas supply device that may be constituted by a bottle bank. The lower part of the buffer tank is provided with one or more outlets 3 which are connected to the normally pressurized part of the pipe system via a quick-acting control valve 7. The pipe system here is illustrated by a section through a ring main 8 distributing water further to a scene of fire via a deluge valve 9 and sprinkle nozzles 11. In Fig. 1 the buffer tank 1 is placed close to a ring main. The functionality of the invention can be maintained also if one chooses to use relatively long pipe stretches between buffer tank and other piping.

The quick-acting control valve 7 is arranged to cooperate with a pilot valve 4 sensing the pressure difference between the upper and lower parts of the buffer tank 1. This pressure difference is a measure of the water level in the buffer tank, and the pilot valve 4 activates a quick closing of the control valve 7 if the pressure difference falls to a given preset level. The activation implies that the control valve is supplied with a fluid under pressure, such that it closes before the water level in the tank is so low that air will be able to enter into the pipe system. In a preferred embodiment said fluid is air which is supplied via a gas regulating valve 5 having good capacity. The control valve 7 will be described below with reference to Fig. 2.

Fig. 2 shows a preferred embodiment of the control valve 7 in open position. This implies that a piston 16 in the valve is pressed maximally away from the valve seat 14 in that the pressure in a control chamber 12 in the valve is lower than the pressure in the pipe system. This will be the normal situation since the buffer tank normally is filled up with water, and the entire fire water system is kept pressurized by the jockey pumps. The control valve 7 will remain in the open position if the pressure in the control chamber 12 is kept lower than the pressure acting on the right pressure surface of the piston 16. When activating the pilot valve 4, the control chamber 12 is supplied with compressed gas, so that the pressure forces against the left pressure surface of the piston exceed the oppositely directed pressure forces from the fire water system. The pressure established in the control chamber 12 is preset to a level P.

When the control valve 7 has reached the closed position, the piston 16 is influenced to a small extent by the pressure in the buffer tank 1. The force trying to push the piston 16 away from the seat 14 essentially is exerted by the pressure in the pipe system. When the fire water pumps start up, the pressure in the pipe system will rise quickly, with the consequence that the piston 16 is pushed away from the seat 14. As shown in Fig. 2, there is provided a pressure relief valve 6 with good capacity, in order to prevent that the pressure in the control chamber 12 rises substantially higher than the pressure P as the control valve 7 is pressed towards the open position. The pressure relief valve thereby closes the control valve to move quickly towards full opening as the pressure in the pipe system rises. In this situation the buffer tank 1 will represent a large elastic volume counteracting that the pressure build-up takes place so quickly that sharp pressure pulses are created.

When the control valve 7 is in the open position, the water may flow as shown with arrows. The piston 16 slides in a cylindrical guide 18 wherein there is milled out three large ports. The guide 18 is connected to the valve housing via three columns 15 forming symmetric divisions between the three ports. In a preferred embodiment the supply line 17 for compressed gas and a lubricating line 13 are placed along two of these columns, to thereby minimize the flow resistance. The lubricating line 13 is connected to a non-illustrated reservoir for a lubricant that is pressed manually into the sealing/sliding surfaces 19 from the outside of the valve, or alternatively is pressed in automatically each time the valve is activated. In Fig. 2 the lubricating line is not placed along a column.

In the preferred embodiment of the control valve said sealing/sliding surface 19 at any time is located in the gas-filled part of the valve. Therefore there is a little risk that the seals are damaged in that some form of fouling takes place in the sliding surface. The lubrication is used to further minimize the risk for this. Figs. 3a and 3b show sectional views of a preferred embodiment of the pilot valve 4 in two different modes. The pilot valve is arranged to be able to open instantaneously for a supply of gas between the gas regulating valve 5 and the control chamber 12 of the control valve 7 as the water level in the tank reaches a given lower limit. In a normal situation this supply is shut off in that the valve body 28 of the pilot valve 4 is pressed down towards a seat 26 and closes the connection between the inlet 25 and the outlet 29. The valve body 28 is fixed to a piston 24. This piston has a cross- section that is larger than the contact surface of the valve body 28 against the seat 26. The pressure influence from the gas that is supplied via the inlet 25, therefore will exert a net downwards-directed force that presses the valve body 28 tightly against the seat 26. To lift the valve body 28 upwards, it is necessary to establish a given pressure at the underside of the piston 24. This will take place momentarily if the pressure against the underside of the illustrated diaphragm 20 falls below a given value, and thereby pulls a valve body 21 which is connected to the diaphragm 20, downwards from its rest against the seat 22. This situation is shown in Fig. 3b. The tension from the spring 30 determines how low water level the tank may have before the pilot valve 4 opens for pressurizing of the control chamber 12 in the control valve 7. When the valve body 21 is pulled away from the seat 22, the pilot valve 4 will immediately go to maximum opening and close the control valve 7 correspondingly quickly. As shown, there is arranged a pair of air bleed channels 23 and 27. These have the task to vent the pressure in the control chamber 12 after the pilot valve 4 again closes. In addition they are to make sure that the control valve 7 cannot close as a consequence of a possible small gas leakage in the installation between the valve bodies 21 and 28 and their respective seat surfaces.

Claims

P a t e n t c l a i m s

1. An arrangement in a fire water system, particularly on an offshore platform, the system including one or more fire water pumps which by way of a pipe system (8) are connected to a number of deluge valves (9) to direct water to respective fire sections, and the arrangement including at least one water-filled buffer tank (1) containing an upper gas-filled volume connected to a gas supply device, characterised in that the gas-filled volume of the buffer tank (1) is connected to the outlet of a high capacity gas supply valve (2) which is connected to the gas supply device and has a preset delivery pressure, and that the buffer tank (1) has a low-placed outlet (3) which is connected to the pipe system (8) by way of a control valve (7) which, cooperating with a pilot valve (4), is arranged to close very quickly if the water level in the buffer tank (1) reaches a lower limit.

2. The arrangement according to claim 1, characterised in that the buffer tank (1) acts as a pulsation damper in that the control valve (7) is arranged to quickly approach its full opening when the fire water pumps start and cause a pressure increase in the pipe system (8).

3. The arrangement according to claim 1, characterised in that the control valve (7) has a piston (16) which is slidably arranged in a cylindrical guide (18) which, together with the piston (16), defines a control chamber (12) when this is pressurized with a fluid, whereby the control valve will close in that the piston is pressed against the valve seat (14).

4. The arrangement according to claim 3, characterised in that the pilot valve

(4) senses the pressure difference between the upper gas-filled volume in the buffer tank (1) and a point far down in the buffer tank, and that the pilot valve (4) is arranged to open for an instantaneous supply of fluid to the control chamber (12) of the control valve (7) if this pressure difference reaches a fixed lower limit.

5. The arrangement according to claim 4, characterised in that said fluid is a gas which is supplied with a preset pressure from a pressure control valve (5) which upstream is supplied with compressed gas from the gas supply device.