GB2334076A - Condensate return pump - Google Patents

Abstract

A condensate return pump for use with a boiler BLR and a condensate reservoir 20 serves to return condensate from the reservoir to the boiler. The pump comprises a double acting drive piston 3 coaxially mounted with two smaller diameter pumping pistons 4, 5 within a housing 2 defining three cylinders. Valves V2, V3, V5, V6 control admission/exhaust of drive vapour (e.g. steam) to the drive piston and valves V1 and V4 control condensate flow. The drive vapour is preferably flashed from the reservoir 20 but may also be 'live' vapour from the boiler (Fig. 4).

Description

Title - Condensate Return Pump This invention relates to a condensate return pump, in particular to a pump for returning condensed water to a boiler in a steam heated process.

In a typical steam heated process, when the steam has given up its latent heat the liquid condensate is discharged through a steam trap. When the condensate passes through the trap, the pressure acting upon it changes from that in the process vessel to a much lower pressure in the condensate return line. The latent heat in the condensate is lower on the process vessel side of the trap than on the return line side. Due to this change, the excess heat causes some of the condensate to flash back into steam on the downstream side of the trap. This flash vapour steam mixes with the liquid condensate and typically flashes off to atmosphere when it reaches the hotwell.

It is known to use a centrifugal / injector type of pump to take condensate from a steam heated vessel or machine under pressure and return this condensate directly back to the boiler/boilers producing the steam. This avoids the heat losses associated with returning the condensate back to a traditional "hotwell" and reduces the amount of make up water required together with its pretreatment costs. However, pumps known for this purpose suffer with problems of cavitation at the impeller and have troublesome glands to seal There has now been devised an improved form of condensate return pump which overcomes or substantially mitigates the above-mentioned disadvantages.

According to the invention, there is provided a condensate return pump for use with a boiler and a condensate reservoir, said pump being operable to draw condensate from the condensate reservoir and return it to the boiler, wherein said pump is driven by vapour generated in the system.

In a preferred embodiment, the pump is driven by flash vapour from the condensate reservoir.

In a preferred embodiment, the pump comprises a cylinder connected to the condensate reservoir and a piston mounted for reciprocating motion within the cylinder, a first stroke of the piston drawing, in use, condensate into the cylinder from the reservoir, and a second stroke expelling said condensate to the boiler.

It is particularly preferred for the piston to be operably linked to a drive piston sliding within a further cylinder, the drive piston being acted upon by flash vapour from the reservoir.

Most conveniently, two pump pistons are provided, each of which is connected to the drive piston such that when one of the pistons draws condensate from the reservoir the other expels condensate to the boiler.

Most conveniently, the two pistons and the drive piston are mounted for coaxial movement and are fixedly joined to form a piston assembly which reciprocates within a pump housing. Such an arrangement is advantageous in that no piston shafts project from the housing, with the consequence that there are no glands to seal and no potential sources of leaks.

Generally, the drive piston (and hence the cylinder within which it reciprocates) will be of greater diameter than the pump pistons. The dimensions of the pistons will be chosen to suit the particular application and the corresponding system pressures, though a range of standard sizes may be suitable for most applications.

Most commonly, the boiler will contain water and the vapour which is generated and which drives the pump will therefore be steam. As noted above, in a preferred embodiment the pump is driven by flash steam from the condensate reservoir. However, the pump may alternatively be driven by "live" steam direct from the boiler.

Live steam may also be used to initiate operation of the process, or if the supply of flash vapour is insufficient to drive the pump, in which case means may be provided for diverting live steam to the pump.

The pump according to the invention is advantageous also in that significant cost savings are achieved by returning hot condensate directly back to the boiler.

A preferred embodiment ofthe invention will now be described, by way of illustration only, with reference to the accompanying drawings, in which Figure 1 is a schematic view of a condensate return pump according to the invention, Figure 2 is a schematic view of the pump of Figure 1, incorporated into a steam heated process, at the commencement of a first operating stroke; Figure 3 shows a view similar to Figure 2 of the pump at the end of the first stroke and commencement of a second stroke; and Figure 4 is a schematic view of an alternative embodiment in which the pump is driven by live steam.

Referring first to Figure 1, a condensate return pump is generally designated 1. The pump 1 comprises a housing 2, the interior of which defines three co-axial cylinders of equal length. The central cylinder is of considerably enlarged diameter relative to the two end cylinders. A drive piston 3 is mounted for reciprocal movement within the central cylinder, and pump pistons 4,5 for similar movement within the end cylinders. The drive piston 3 and pump pistons 4,5 are fixedly mounted on a common shaft 6.

The piston defines four chambers A,B,C,D within the housing 2. Each of the central chambers B,C has a flash steam inlet 7,8 and a flash steam exhaust 9,10. The flash steam inlets 7,8 and exhausts 9,10 are controlled by valves V2,V3,V5,V6.

The end chambers A,D have condensate inlets 11,12 and condensate outlets 13,14. The condensate inlets 11,12 are controlled by valves V1,V4 and the condensate outlets are fitted with non-retum valves 15,16.

The pump 1 is connected to a flash steam vessel 20, a boiler BLR and hotwell H as shown in Figure 2. Flash steam from the vessel 20 is led selectively to chamber A or chamber D, and subsequently exhausted to the hotwell H. Condensate from the vessel 20 can be drawn into either chamber B or chamber C and then positively displaced to the boiler BLR.

The operating sequence of the pump 1 is as follows: First Stroke Valves V3, V4 and V5 open and valves V1, V2 and V6 remain shut.

Chamber C fills with flash steam at 40 psi through valve V3 and starts moving the pistons 3,4,5 to the left (as viewed in the drawings).

Chamber D fills with liquid condensate at 40 psi through valve V4.

The combined pressures and effective piston areas move the liquid condensate in chamber A through the non-return valve 15 and against a pressure of approximately 170 psi into the boiler BLR with a working pressure of 150 psi.

During this operation chamber B is exhausted to the hotwell 4 via valve V5.

Second Stroke Valves V1,V2 and V6 open and valves V3,V4 and V5 shut.

Chamber B fills with flash steam at 40 psi through valve V2 and starts moving the pistons 3,4,5 to the right.

Chamber A fills with liquid condensate at 40 psi through valve V1.

The combined pressures and effective piston areas move the liquid condensate in chamber D through the non-return valve 16 and against a pressure of approximately 170 psi into the boiler BLR with a working pressure of 150 psi.

During this operation chamber C is exhausted to the hotwell 4 via valve V6.

The vessel 20 is provided with a 45 psi relief valve 30 to vent surplus flash steam to the hotwell H.

If insufficient steam is flashed off to drive the pump 1, live steam may be introduced via a reducing valve 40.

The valves V 1 -V6 may be electrically or pneumatically operated, the sequential operation of the valves preferably being under computer control. Appropriate sensors may be provided to detect the completion of the first and second strokes of the pistons 3,4,5.

Operation of the pump 1 in the manner described has the effect of positively displacing hot condensate back to the boiler BLR. This leads to significant cost savings, particularly since (apart from the power necessary to operate the valves and run any associated control circuitry) the system provides its own energy by utilising the flash steam from the vessel 20. The result of the diminished heat loss achieved using the invention is that for any given process the fuel costs are reduced. It will be understood that the pressures etc referred to above are for illustrative purposes only as the system operating pressures and other parameters will vary.

Finally, Figure 4 shows a schematic view of an alternative system according to the invention in which the pump is driven by live steam direct from the boiler. For the most part this embodiment is similar to that of Figures 2 and 3 and corresponding components are indicated by the same reference numerals. In this case, however, the drive piston 3 is driven not by flash steam from the flash steam vessel 20, but by live steam fed via a steam line 60 through either valve V2 or V3 to the respective central chamber B,C ofthe housing 2. A conventional steam trap 50 is fitted to pass any excess condensate or flash steam to the hotwell H to deal with any temporary overload.

Claims (9)

1. Claims 1. A condensate return pump for use with a boiler and a condensate reservoir, said pump being operable to withdraw condensate from the condensate reservoir and return it to the boiler, wherein said pump is driven by vapour generated in the system.
2. 2. A pump as claimed in Claim 1, wherein the pump is driven by flash vapour from the condensate reservoir.
3. 3. A pump as claimed in Claim 1 or Claim 2, wherein the pump comprises a cylinder connected to the condensate reservoir and a piston mounted for reciprocating motion within the cylinder, a first stroke of the piston drawing, in use, condensate into the cylinder from the reservoir, and a second stroke expelling said condensate to the boiler.
4. 4. A pump as claimed in any preceding claim, wherein the piston is operable linked to a drive piston sliding within a further cylinder, the drive piston being acted upon by flash vapour from the reservoir.
5. 5. A pump as claimed in Claim 4, wherein two pump pistons are provided, each of which is connected to the drive piston such that when one of the pistons draws condensate from the reservoir the other expels condensate to the boiler.
6. 6. A pump as claimed in Claim 5, wherein the two pistons and the drive piston are mounted for coaxial movement and are fixedly joined to form a piston assembly which reciprocates within a pump housing.
7. 7. A pump as claimed in any preceding claim, wherein the vapour is steam.
8. 8. A pump substantially as hereinbefore described and as illustrated in Figures 1 to 3.
9. 9. A pump substantially as hereinbefore described and as illustrated in Figure 4.