GB2147050A

GB2147050A - Liquid ring pumps

Info

Publication number: GB2147050A
Application number: GB08424369A
Authority: GB
Inventors: Keith Mangnall
Original assignee: Hick Hargreaves and Co Ltd
Current assignee: Hick Hargreaves and Co Ltd
Priority date: 1983-09-27
Filing date: 1984-09-27
Publication date: 1985-05-01
Also published as: GB2147050B; GB8325813D0; GB8424369D0

Abstract

A vacuum maintaining system for a steam turbine condenser 14 comprises a first compression stage including at least one steam-operated vacuum augmentor 10, 11 connectible to a main condenser and communicating with the water-cooled subsidiary condenser 14, and a second compression stage which includes a liquid ring vacuum pump 18 supplied with service water from a tank 19. A centrifugal air/water separator 41 is mounted on the tank to receive and separate the air/water discharge from the liquid ring vacuum pump, and a heat exchanger 21 is positioned in the tank for controlling the temperature of the service water supplied via a pipe 20 to the liquid ring vacuum pump. An air outlet of the subsidiary condenser 14 is connected to an air-operated ejector 57 located in a suction branch of the liquid ring vacuum pump to serve as a ballasting device to maintain an acceptable air load for the vacuum pump, for example in the event of one or both vacuum augmentors 10, 11 being inoperative. <IMAGE>

Description

SPECIFICATION A vacuum maintaining system The present invention relates to a vacuum maintaining system for condensers.

Over the past few years problems have been encountered in many major power stations in relation to the availability, operational performance and reliability of turbine-condenser vacuum plants. These problems stem directly from the adoption of mechanical vacuum pumping systems to replace the traditional steam operated ejectors in the interests of energy conservation, together with certain shortcomings in the design and construction of equipment containing the various airspaces to be evacuated. This latter factor has manifested itself in excessive air in-leakages which require consequently increased air extraction capacity from the vacuum plant.

Unfortunately, this and other problems have shown the mechanical pumping systems to be relatively inflexible and prone to failure under conditions of transient loading and operation outside the original design parameters.

Another significant factor in plant failure has been due to the use of "coldest source" water in vacuum pump service water systems.

This water is usually seawater and its use has led to coorosion problems, not only in the general metallurgual sense, but also as a result of salt carry-over from direct-contact spray condensers.

It is an object of the present invention to provide a vacuum maintaining system for condensers in which the above mentioned problems are substantially obviated or mitigated.

According to the present invention there is provided a vacuum maintaining system for a condenser comprising a first compression stage including a steam-operated vacuum augmentor connectible to a main condenser and communicating with a water-cooled subsidiary condenser, and a second compression stage including a liquid ring vacuum pump supplied with service water from a circulating tank on which is mounted, in communication therewith, an air/water separator adapted to receive and separate the air/water discharge from the liquid ring vacuum pump, and in which is disposed a heat exchanger for controlling the temperature of the service water supplied to the liquid ring vacuum pump.

Also according to the present invention there is provided a heat exchanger, especially but not exclusively for a condenser vacuum maintaining system, comprising a plurality of spaced baffle plates defining a sinuous flow path for one fluid, and, between adjacent baffle plates, at least one tube with the tubes being interconnected to provide a contra-flow path for a second fluid at a different temperature to the first fluid.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic view of a vacuum maintaining system for a condenser according to the present invention; Figure 2 is a perspective view of the condenser vacuum maintaining system; and Figures 3 and 4 are respectively longitudinal and transverse section views of a heat exchanger/water supply tank arrangement.

The condenser vacuum maintaining system comprises a duty steam-operated vacuum augmentor 10 and a larger stand-by steam-operated vacuum auk mentor 11 supplied via valve-controlled piping 1 2 with operating steam, generally waste steam from a low pressure source (not shown) and, if necessary, from an auxiliary boiler (also not shown). The auk mentor 11 provides increased capacity should adverse air leakage occur in the system.

Air/vapour, possibly with entrained noncondensables, from a turbine condenser (not shown), say operating in a power station, are delivered to the aug mentors 1 0, 11 via valvecontrolled piping 1 3.

The operating steam and vapour/air with any non-condensables are discharged, under normal duty conditions, by the augmentor 10 only into the shell side of a surface condenser 1 4 of the floating head type which is supplied with cooling water via a pipe 15, say, from the power station, or any other cold water supply. This surface condenser 14 is desirably constructed in materials which conform to the metallurgy of the turbine condenser. For example, if the system is being used in a power station, it may have titanium tubes and titanium-clad tubeplates.

Any accumulated condensate in this stage of the system is drained from the surface condenser 1 4 back to the turbine condenser via a pipe 16, or it may alternatively be diverted via a pipe 1 7 to provide a make-up liquid supply for a liquid ring vacuum pump 1 8 which forms part of the second compression stage of the vacuum maintaining system.

This liquid ring vacuum pump 1 8 is a twostage unit directly driven by a motor 18A and which is supplied with water from a service water recirculating tank 1 9 via valve-controlled piping 20.

The recirculating tank 1 9 incorporates a heat exchanger 21 which will be described in more detail later with reference to Figs. 3 and 4. Cold or cooling water is supplied to the heat exchanger 21 from the pipe 15 via a branch pipe 22 and exits from the heat exchanger 21 via a pipe 23 to which is connected a branch pipe 1 SA from the surface condenser 14 for its exiting cold water.

Service water is supplied. by way of makeup, to the recirculating tank 1 9 via a valvecontrolled pipe 24.

Reference is now conveniently made to Figs. 3 and 4.

The recirculating tank 1 9 comprises spaced weir plates 25, 26 and 27 with the heat exchanger 21 located between the weir plates 25 and 26. Weir plates 25 and 27 are fixed to the bottom of the tank 1 9 while intermediate weir plate 26 is spaced above the bottom and is higher than the other two.

These weir plates 25 to 27 create a differential service water pressure head which causes the service water to flow to liquid ring pump delivery pipe 20. The water level in the tank 1 9 upstream of the weir plate 26 is higher than the water level downstream thereof with the heat exchanger 21 being at the upstream side of the weir plate 26.

The heat exchanger 21 comprises a fixed tube plate 28 and a floating tube sheet 29 between which extend baffle plates 30 which define a sinuous descending flow path for the service water from the upstream side of the weir plate 26 to the downstream side thereof (see arrows 31 in Fig. 3).

Above the upper baffle plate 30 and between adjacent baffle plates 30 are disposed tubes 32 communicating at each end with a water box 33, 34.

Cooling water enters the lower end of water box 33 and flows upwardly back and forth through the tubes 32, the water box 34 and the water box 33 to exit from the top of the latter via the pipe 23.

The water boxes 33, 34 are compartmentalised by division walls 33A, 34A respectively to provide this ascending to-and-fro flow indicated by arrows 35.

The cooling water flow is both normal to and counter to the service water flow as will be manifest from the above.

The floating end water box 34 is connected to the tank 1 9 by a floating head diaphragm seal 36 and is connected thereto by bolts or other releasable connectors (not shown).

The water tube/baffle plate pack 30, 32 is mounted on rollers 37 supported on rails 38 within the tank 1 9 so that with the floating end water box 34 removed the pack can be removed for inspection and cleaning.

The heat exchanger 21 is, in effect, a surface cooler whose function is to ensure a constantly acceptable service water temperature for the liquid ring vacuum pump 18.

Merely as an example, the tank 1 9 is formed of epoxy-lined carbon steel, the tubes 32 are formed of titanium, the tube plate 28 and tube sheet 29 are titanium clad, the water boxes of ceilote-lined carbon steel and the rollers 37 are of nylon.

To the upper and lower compartments of the water box 33 are connected thermopockets 39 mounting thermocouples 40 to permit measurement and indication of inlet and outlet cooling water temperatures.

To complete the description of the recirculating tank there is connected to the upper end thereof, on the upstream side of the weir plate 26 an air/water separator 41 (a centrifugal one in this instance). This has an upper air vent 42 surmounted by a cowl 43 and at its bottom opens into the tank 1 9 as indicated at 44 (Fig. 3). The discharge from the liquid ring vacuum pump 1 8 is delivered by a pipe 45 to an inlet 46 of the centrifugal air/water separator 41 which centrifuges this air/water discharge into an upward air flow (arrow 47) and a downward water flow (arrow 48). The water flow 48 passes through a vortex breaker 49 just before entering the tank 1 9.

Thus non-condensibles are vented to atmosphere while hot service water is delivered back into the tank 1 9 for cooling by the heat exchanger 21 prior to return to the liquid ring vacuum pump 18.

50 designates a connection point on the air vent of the separator 41 for receiving a flow measuring instrument (not shown).

Overflows from the tank 1 9 are designated 51. These obviate flooding of the tank compartments defined by the weir plates 25 to 27 to maintain a safe service water level in the liquid ring vacuum pump 1 8 after shutdown since it is well known that inertia losses set up when starting with a completely flooded pump can produce serious internal shock damage.

It is desirable that the pump structural materials be suitable for an arduous operating environment and, again merely by way of example, it is preferred to use aluminium bronze impellors and shaft sleeves, a stainless steel shaft and an S.G. cast iron casing and guide plates.

The tank 1 9 is provided with drains as indicated at 52, and connections 53 and 54 respectively for a conductivity probe and a differential temperature switch. A level warning alarm 55 is also provided for the tank 1 9.

Inspection covers are indicated at 56.

The failure of liquid ring vacuum pumps due to the effect of the cavitation phenomenon is well known. The vacuum maintaining system according to this invention incorporates liquid ring vacuum pump protection in the form of cavitation prevention measures, generally being caused either by under-loading of the liquid ring vacuum pump so that it operates at an unsafe vacuum level, or by using service water at a temperature considerably above design whilst attempting to maintain design vacuum.

According to this invention there is fitted on the suction branch of the liquid ring vacuum pump 1 8 and connected to the surface condenser air outlet is an atmospheric air-operated ejector 57. Its principal function is to act as a ballasting device to maintain an acceptable air load for the vacuum pump 1 8 under the most arduous conditions, such as the steam augmentor 10 (or 10 and 11) being out of commission. This prevents unsafe esca lation of the vacuum level at the vacuum pump 18. However, the ejector 57 is also designed to utilise atmospheric air as a motive gas in order to achieve a degree of compression from its suction to its diacharge, thus assisting the vacuum pump 1 8 to maintain a slightly higher vacuum upstream.The ejector operating air is therefore taken from the air vent 42 of the separator 41 via a pipe 58.

This is done to eliminate possible interference and to ensure a clean air supply.

It is to be noted that if a total cold water supply failure to the heat exchanger were to occur, the system would continue to run while gradually losing water by evaporation due to the heat generated in the operation of the vacuum pump 5. To guard against plant failure under this condition, the tank 1 9 is, as aforesaid, fitted with low liquid level alarm and temperature alarm devices, the vacuum 1 8 then being operated on a continuous make-up water basis (pipe 24).

The provision of an integral surface cooler is not only cost-effective in relation to a separate heat exchanger and its pumping problems but it also eliminates the possibility of blockage due to debris and is obviously space saving.

In order to minimise floor space requirements for the vacuum maintaining system, it can be supplied wholly or partially skidmounted in various configurations to suit individual installations.

The heat exchanger disclosed herein may have applications other than in the abovedescribed condenser vacuum maintaining system.

Claims

1. A vacuum maintaining system for a condenser comprising a first compression stage including a steam-operated vacuum augmentor connectible to a main condenser and communicating with a water-cooled subsidiary condenser, and a second compression stage including a liquid ring vacuum pump supplied with service water from a circulating tank on which is mounted, in communication therewith, an air/water separator adapted to receive and separate the air/water discharge from the liquid ring vacuum pump, and in which is disposed a heat exchanger for controlling the temperature of the service water supplied to the liquid ring vacuum pump.

2. A system according to claim 1, in which an air outlet of the subsidiary condenser of the first compression stage is connected to an air-operated ejector located in a suction branch of the liquid ring vacuum pump of the second compression stage and which serves as a ballasting device to maintain an acceptable air load for the vacuum pump.

3. A system as claimed in claim 1 or 2, comprising a second and standby steam-operated vacuum augmentor for connection to the main condenser and in communication with the subsidiary condenser.

4. A system as claimed in any one of claims 1 to 3, in which the liquid ring vacuum pump is a two-stage unit and is directly driven.

5. A system as claimed in any one of claims 1 to 4 comprising a common cold water supply to the subsidiary condenser and the heat exchanger and a common outlet therefor.

6. A system as claimed in any one of claims 1 to 5, comprising piping connected to the subsidiary condenser for ducting condensates therefrom either to the main condenser or to the liquid ring vacuum pump.

7. A system as claimed in any one of claims 1 to 6, in which the circulating tank comprises weir plates adapted to create a differential pressure head in the service water therein to ensure flow of service water from the tank to the liquid ring vacuum pump.

8. A system as claimed in claim 7, comprising a service water make-up feed connected to the circulating tank.

9. A system as claimed in any one of claims 1 to 8, in which the air/water separator is a centrifugal one which centrifuges the discharge from the liquid ring vacuum pump to allow air and non-condensables to vent to atmosphere and water to return to the circulating tank.

10. A system as claimed in claim 9, in which the air-operated ejector is operated by atmospheric air and is connected to the air vent of the air/water separator.

11. A system as claimed in any one of claims 7 to 10, in which the heat exchanger is in the form of a surface cooler and is disposed at the service water side of the circulating tank, i.e. on the upstream side of the differential pressure head created by the weir plates.

1 2. A system as claimed in claim 11 in which the heat exchanger comprises a plurality of spaced baffle plates defining a sinuous flow path for one fluid, and, between adjacent baffle plates, at least one tube with the tubes being interconnected to provide a contra-flow path for a second fluid at a different temperature to the first fluid.

1 3. A system as claimed in claim 12 in which the baffle plates are vertically spaced with a plurality of tubes between adjacent baffle plates and above the top baffle plate.

14. A system as claimed in claim 13, in which the tubes are open at their ends into compartmentalised water boxes, the arrangement being that the second fluid flow is sinuously ascending.

15. A system as claimed in claim 13 or 14, in which the baffle plates are arranged to provide a sinuous descending flow path for said one fluid which flow path is normal to the second fluid flow path.

16. A system as claimed in any one of claims 1 2 to 1 5 in which the baffle plate/ tube pack is withdrawable from the circulating tank for inspection and maintenance purposes.

1 7. A system as claimed in claim 1 6 in which the tubes and baffle plates extend between a fixed tube plate and associated water box and a floating tube sheet and associated water box.

1 8. A system as claimed in claim 1 7 in which the floating tube sheet is releasably connected to the circulating tank via a diaphragm seal and supports the baffle plates and tubes.

1 9. A vacuum maintaining system for a condenser substantially as hereinbefore described with reference to the accompanying drawings.

20. A heat exchanger, especially but not exclusively for a condenser vacuum maintaining system, comprising a plurality of spaced baffle plates defining a sinuous flow path for one fluid, and, between adjacent baffle plates, at least one tube with the tubes being interconnected to provide a contra-flow path for a second fluid at a different temperature to the first fluid.

21. A heat exchanger as claimed in claim 20, in which the baffle plates are vertically spaced with a plurality of tubes between adjacent baffle plates and above the top baffle plate.

22. A heat exchanger as claimed in claim 20 or 21 in which the tubes are open at their ends into compartmentalised water boxes, the arrangement being that the second fluid flow is sinuously ascending.

23. A heat exchanger as claimed in claim 21 or 22, in which the baffle plates are arranged to provide a sinous descending flow path for said one fluid, which flow path is normal to the second fluid flow path.

24. A heat exchanger as claimed in any one of claims 20 to 23, in which the baffle plate/tube pack is withdrawable from the circulating tank for inspection and maintenance purposes.

25. A heat exchanger as claimed in claim 24, in which the tubes and baffle plates extend between a fixed tube plate and associated water box and a floating tube sheet and associated water box.

26. A heat exchanger as claimed in claim 25, in which the floating tube sheet is releasably connected to the circulating tank via a diaphragm seal and supports the baffle plates and tubes.

27. A heat exchanger, especially but not exclusively for a condenser vacuum maintaining system, substantially as hereinbefore described with reference to the accompanying drawings.