US2904018A

US2904018A - Three-element feedwater control

Info

Publication number: US2904018A
Application number: US669137A
Authority: US
Inventors: Igor J Karassik
Original assignee: Individual
Current assignee: Individual
Priority date: 1957-07-01
Filing date: 1957-07-01
Publication date: 1959-09-15
Anticipated expiration: 1976-09-15

Description

Sept. 15, 1959 1. J. KARASSIK THREE-ELEMENT FEEDWATER CONTROL 2 Sheets-Sheet 1 Filed July 1, 1957 Liam Dmmu m .om

ML-(MI IFUKPFZOU LIUUNID IGORIKARASSIK INVENTOR. By W X7 Sept. 15, 1959 J. KARASSIK 2,904,018

THREE-ELEMENT FEEDWATER CONTROL Filed July 1, 1957 2 Sheets-Sheet 2 4 n a r 5 ll) (I 6'! V) N/ E I A M g f l- M $5 1t 5 a i- I N D it a A a w M N g:

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IGOR J. KARASSIK FIG. 2

United sfilt s v 2304'318 I p v 'lHRE-EiELEME-NI FEEnwATER. common 7 Igor I. Karasslk, lllaplewot'itl NJ. Application July 1s,.1i9s7,{Sens1 -1-1o.669 1s1 Y 6" claims. (Cline-451p This. invention relates to meanstprovidingz automatic protection against. damage to" a: boiler feed pumps More particularly the invention: contemplates the. con trol'. of. the deliver-yof feedwater firom the boiler. feed pump in such a manner as to substantially preclude the possibility of reducing the available net positive suction head at the suction of'the boiler feed pump taking its suctionfromadirect contact heater supplied with steam from a bleed stage of the main turbine.

Boiler feed'water systems. usually include the standard three-element control for regulating the new oflfeedwater to the boiler. This three-element control includes a steam. flow meter, a feedwater flowmet'er and the usual boiler water level measuring'd'cvice. I Steam flow from the boiler drum. to thetlirbiiie and water flow from the pump to the boiler druni are a curately measured by the steam fibvv meter and feedwater flow meter respectively. These rn'eters'are generally balanced against each other by equating. nieanslsuch as. a ratio relay -or a differential ontrol linkage so that the quantity orteed'wat'er supplied to the is nearly equal to the quantity of steam output therefrom.

Should the balance be disturbed, as for example, by an increase iii steam demand; the differential control linkage or the' ratio relay will 'sig'naliei t'her a feedvvat'ei control valve or a control means for varying the speed oif fitlie ously reduced.-

pump to provide the necessary increased flilid' flowto the I I boiler}! While the majority of lar e boiler feed'water installations are provided withthree-element feedwater' controls, there are variations as to the manner iii which these controls. are adjusted". In general. the feedwater input is regulated to closely. parallel steam output, but the threeel mentcontrol is sometimes adjusted. to vary the boiler w ter level with the load as a precaution against the efroots. of swell and shrinkage. The water level measuring device operates, in the foregoing installation, to superimpose its influence upon the signal fronr'the' diifer'ential control linkage or: ratio relay'to actuate, usually, a feedwater control valve to makethe' necessary readjustments in rate of feedwaterflow to the Water drum. I

Under conditions met in actual service,. it has been noted that the foregoing control does not provide the desired fee'dwatcr' new rate regulation during a large or sudden drop in the main turbine load; I I

Specifically, the various effects taking place on a surden' or large reduction in turbine load" are as follows:

The turbine governor reduces steam flow in order to maintain the proper relation to the generated power; this reduction results in a proportionate pressure reduction in all or the successive stages" of the turbine, including the bleed stage supplying steam to the direct contact heater.

The extraction line to the direct contact heater is usuaHy provided witha check valve; The heater, being areservoir of hot water; cannot reduce pressure as rapid 'l y'as the'turbinc bleed stage; therefore; the valve closes,

isolating the heater from the turbine untilthe pressure 7 Patented Sept. 15,1959

has dropped. to-that of the bleed point for the new load. As a consequence. the net. positive suction. head available for the boiler feed. pump, due to .the changing relationship between the decaying. heater pressure and the feedwater temperaturev at. the suction of the pump, is. seri- Specifically, the foregoing. reduction. in. available net positive suction head occurs because of a decrease in the margin between the suction pressure and the vapor pressure corresponding. to the fluidtemperature at the pump suction. when. the, mainturbine load is. reduced rapidly. The heater ressure, which is acomponent of the suction pressure, drops during, such a reduction without a corresponding" immediate feedwaer' temperature reduction at the pum suction, thereby creating flashing in the suction components oi'the boiler reed ump which in efiect reduces the available not positive suction head. Obviously theforegoing reduction of the net positive suct'ion he'ad may have adverse. effects on the com onent parts or a boiler feed pump. and iris a parentthatserious damage may result to the pump if there is an increase in; the required fl w to the boiler under; these conditions. Concomitant with. the reduction in availablenet posi tive suction head there occurs, due to the reduction in steam flow from the boiler water drum, an increase in the pressure in the boiler water drum. This-increase in pressure causes the bubbles in the drum to collapse and therefore indicate an apparent drop in the level of the water thereinto the waterlevelmeasurin device.

Accordingly; the water level measuring device. signals forani crease in new to the water drunr, which increase inflow has a detrimental effector! the boilerfee rpunip because of the lack ofsuflicient net positive suction head resulting" from thei'solation of the direct contact" heater as a consequence of jthe'd'ro in the m in turbine. load.

Inasmuch as tire capacityof a boiler fed'pllmp operating inapower plant system issoielytgoverned by the three-element control which is in turn regul ted by the fluctuations; in theniain turbine load and the resulting rise or fail of t-he'- boiler level, some specific means. must be applied to-nullify'tl'ie signa-l' fronr the water level measuring deviceduriiiga period of large or' suuden load drop in'the' turbine. I

This is accomplished in the present invention by a three-element control including a pneumatic means to transmit the impulses-from the steam flow meter;. a feedwa-terfl'ow' rnet'er and a level controllerto'tlre'flowiate controlling mechanism; the direct contactjheater being further provided with adjustable measuring means to nullity the signal from the level control to the flow rate control.

Accordingly. it is an object of' this invention to providea direct contact heater with means to-nullif'y the signal from awater level measuring device when the pressure in the directcontact heater is decreasingat a rate inexcess ofi'a prodrtermined value.

While the invention will be described and shown'in connection with a pneurllatic system for transmitting the signals from the foregoing controls to the flow rate cont-rol: means; it is not-intended; to so limit the invention, which: may be applied' advantageously to hydraulically operated control; systems or elteciironicv control; systems or the various features: of construction andcombination of parts which will become apparent from the detailedd'ee scription in connectionwith the accompanying. drawings Figure 2 shows. the measuring m eans for the signal of the water level" measures in a boiler. feed water installation wherein water now is controlled therethrough by adjusting the speed of the prime mover of Y a boiler feed pump.

Referring to the drawings, Figure 1 shows a typical open cycle boiler ,feedwater'installation which generally includes: a direct contact heater 1 'aboiler feed pump 2, ;a boiler water drum 3, a main" turbine (not shown),

andthe usual feedwater regulation controls to be hereinafter more fullydescribed.

" The direct contact heater 1 receives water through condensate line 4, which line communicates at the other end with a condensate pump, not shown, in the drawings. The water contained in the direct contact heater 1 being heated, and as a consequence pressurized, by steam supplied thereto through bleed line 5; which line communicatesat the other end with a bleed stage of the main turj' The boiler feed pump 2 draws conditioned water from the 1 direct contact heater 1 and transmits the water through discharge line 6 to' the boiler water drum 3 of the feedwater installation.

The boiler water drum 3 is adapted to accommodate water in the lower portion 7 and steam in the upper portion 8. The steam being highly pressurized flows through steam line 9 to the main turbine for operation thereof. Automatic feedwater flow regulation controls are customarily provided :for feedwater installations to meet the exacting requirements of present day boilers.

The feedwater regulator controls comprise: a feedwater flow meter 10 for measuring the flow rate through discharge line 6, a steam 'flow meter 11 for measuring the flow through steam line 9, a water level measurer 12 for measuring the level in water drum 3, a check or nonreturn valve 13 in bleed line 5 for precluding the return fromdirect contact heater 1 to the main turbine, measuring-means 14, such as an adjustable pressure rate relay, connected to the direct contact heater 1 for measuring the', pressure therein and for nullifying the impulse of the, .water level .recorder 12, which impulse is transmitted to a'flow controlling device 15, .such as a feedwater regulating valve, when the pressure in the direct contact heater 1 is, decreasing in excess of a predetermined rate, and the usual impulse transmitting means generally designated '16 as forexample, a pneumatic. system for transmitting from the foregoing measuring controls to the flow controlling device 15 to provide proper flow in the *feedwater installation under most all conditions.

The feedwater flow meter 10 available on the open market is connected to either side of an orifice 17 disposed .in discharge .line 6 by pressure lines 18 and 19. Similarly, the steam flow meter 11 is connected to either .sideof orifice 20, disposed in steam line 9, by pressure lines 21and 22.

Flow meter. 10 'and steam flow meter 11 are connected to one another as, for example, by a differential control linkage 23 so that the feedwater flow closely parallels the steam flow. Should the flow in steam line 9 deviate from the predetermined ratio, the differential linkage will actuate the flow controlling device 15, through impulse transmitting means 16, to increase or decrease the flow rate discharge line 6 to maintain the system in equilibrium'or at the predetermined ratio.

In addition, to insurethc proper water drum level, a

water level measurer '12 is provided for the water drum 3 and connected thereto through

lines

24 and 25. An improper level impulse being transmitted through a junction box 26, such as a ratio relay and impulse transmitting means 16 to the flow controlling device 15 to make the proper readjustments in water level.

v The usual impulse transmitting means 16 in a boiler feedwater installation comprises a pneumatic system provided with an air supply 27, pilot valves 28, an air operated control relay 29 sold under the trademark Standatro and a selection box 30 permitting manual or autolmatic flow control of the feedwater.

4 Operation In operation, an increase or decrease in steam flow through steam line 9 will vary the airloading pressure in pilot valve 28a above or below the neutral set, through differential control linkage 23. The variation is a result of a change in load demand; accordingly, an impulse is transmitted through Standatrol 29 and selection box 30 to flow control device 15 so that the flow is adjusted in feedwater line 6 to restore the predetermined balance between the flow meters 11 and 12.

The water level recorder 12 will actuate the flow controlling device 15 through junction box 26, Standatrol 29 and selector box 30 to adjust the flow to insure the proper water drum level in boiler water drum 3.

The adjustable measuring means 14, for example, a pressure rate relay, is connected to the direct contact heater 1 through line 31 for recording the pressure therein.-' An impulse being transmitted through pilot valve 28b, and line 32 to junction box 26 to nullify the signal from the water level recorder 12 when the pressure in the direct contact heater 1 is decreasing in excess of a predetermined value.

Modifield form Figure 2 of the drawings shows a modified form of the improved three-element control. Those parts having generally the same construction and purpose as the corresponding parts in Figure 1 have been given the same reference characters.

Instead of having a feedwater regulating valve 15 for metering the flow to the boiler, Figure 2 provides a repositioning means 15a such as a servomotor to control the throttling of the turb'me 29 driving the boiler feed pump 2, and instead of the diflerential control linkage 23 for signaling for an increase or decrease in flow, Figure 2 provides a ratio relay 23a for maintaining the predetermined balance between the feedwater flow meter and the steam flow meter.

It will be understood that the invention is not to be limited to the specific construction or arrangement of parts shown, but that they may be widely modified within the invention defined by the claims.

What is claimed is:

1. Control means for a boiler feed system including'a boiler for supplying power medium, a boiler feed pump for flowing fluid medium to said boiler, a direct contact heater for supplying fluid to said boiler feed pump, said control means comprising measuring means for measuring the flow of the power medium from the boiler, second flow measuring means between the boiler feed pump and the boiler, balancing means operatively connected to said first and second measuring means, flow controlling means in said system operatively connected to said balancing means and adapted to vary the flow of said boiler feed pump to said boiler, a water level measurer for said boiler operatively connected to said flow control means whereby said water level measurer will signal said flow control means to increase or decrease the flow to said boiler to hold the fluid medium therein between predetermined limits, and pressure measuring means connected to said direct contact heater and operatively connected to said Water level measurer to nullify the signal transmitted from said water level measurer to said flow control means when the pressure in said direct contact heater is being reduced at more than a predetermined rate.

2. Control means for a boiler feed system including a boiler for supplying power medium, a boiler feed pump for flowing fluid medium to said boiler, a direct contact heater for supplying fluid to said boiler feed pump, said control means comprising measuring means for measuring the flow of the power medium from the boiler, second flow measuring means between the boiler feed pump and the boiler, balancing means operatively connected to said fi rst and second measuring means, flow tlntrolling means in said system adapted to vary the flow of said boiler feed pump to said boiler, impulse transmitting means operatively connected to said balancing means and said flow controlling means whereby a signal from said balancing means will be transmitted through said impulse transmitting means to said flow controlling means, a water level measurer for said boiler operatively connected to said impulse transmitting means whereby said water level measurer will signal said flow control means to increase or decrease the flow to said boiler to hold the fluid medium therein between predetermined limits, and pressure measuring means connected to said direct contact heater and operatively connected to said water level measurer to nullify the signal transmitted from said water level measurer to said flow control means when the pressure in said direct contact heater is being reduced at more than a predetermined rate.

3. Control means for a boiler feed system including a boiler for supplying power medium, a boiler feed pump for flowing fluid medium to said boiler, a direct contact heater for supplying fluid to said boiler feed pump, said control means comprising measuring means for measuring the flow of the power medium from the boiler, second flow measuring means between the boiler feed pump and the boiler, balancing means operatively connected to said first and second measuring means, flow controlling means in said system adapted to vary the flow of said boiler feed pump to said boiler, impulse transmitting means operatively connected to said balancing means and said flow controlling means whereby a signal from said balancing means will be transmitted through said impulse transmitting means to said flow controlling means, a water level measurer for said boiler operatively connected to said impulse transmitting means whereby said water level measurer will signal said flow control means to increase or decrease the flow to said boiler to hold the fluid medium therein between predetermined limits, and an adjustable pressure rate relay connected to said direct contact heater and operatively connected to said water level measurer to nullify the signal transmitted from said water level measurer to said flow control means when the pressure in said direct contact heater is being reduced at more than a predetermined rate.

4. Control means for a boiler feed system including a boiler for supplying power medium, a boiler feed pump for flowing fluid medium to said boiler, a direct contact heater for supplying fluid to said boiler feed pump, said control means comprising measuring means for measuring the flow of the power medium from the boiler, second flow measuring means between the boiler feed pump and the boiler, balancing means operatively connected to said first and second measuring means, flow controlling means in said system operatively connected to said balancing means and adapted to vary the flow of said boiler feed pump to said boiler, a junction box operatively connected to said flow controlling means, a water level measurer for said boiler operatively connected to said junction box whereby said water level measurer will signal said flow control means through said junction box to increase or decrease the flow to said boiler to hold the fluid medium therein between predetermined limits, and an adjustable pressure rate relay connected to said direct contact heater and operatively connected to said junction box to nullify the signal transmitted from said water level measurer to said flow control means when the pressure in said direct contact heater is being reduced at more than a predetermined rate.

5. Control means for a boiler feed system including a boiler for supplying power medium, a boiler feed pump for flowing fluid medium to said boiler, a direct contact heater for supplying fluid to said boiler feed pump, said control means comprising measuring means for measuring the flow of the power medium from the boiler, second flow measuring means between the boiler feed pump and the boiler, balancing means operatively connected to said first and second measuring means, flow controlling means in said system adapted to vary the flow of said boiler feed pump to said boiler, impulse transmitting means operatively connected to said balancing means and said flow controlling means whereby a signal from said balancing means will be transmitted through said impulse transmitting means to said flow controlling means, a ratio relay operatively connected to said flow controlling means, a water level measurer for said boiler operatively connected to said ratio relay whereby said water level measurer will signal said flow control means to increase or decrease the flow to said boiler to hold the fluid medium therein between predetermined limits, and pressure measuring means for said direct contact heater and operatively connected to said ratio relay to nullify the signal transmitted from said water level measurer to said flow control means when the pressure in said direct contact heater is being reduced at more than a predetermined rate.

6. Control means for a boiler feed system including a boiler for supplying power medium, a boiler feed pump for flowing fluid medium to said boiler, a direct contact heater for supplying fluid to said boiler feed pump, said control means comprising measuring means for measuring the flow of the power medium from the boiler, second flow measuring means between the boiler feed pump and the boiler, balancing means operatively connected to said first and second measuring means, flow controlling means in said system adapted to vary the flow of said boiler feed pump to said boiler, impulse transmitting means operatively connected to said balancing means and said flow controlling means whereby a signal from said balancing means will be transmitted through said impulse transmitting means to said flow controlling means, a ratio relay operatively connected to said flow control means, a Water level measurer for said boiler operatively connected to said ratio relay whereby said water level measurer will signal said flow control means to increase or decrease the flow to said boiler to hold the fluid medium therein between predetermined limits, and an adjustable pressure rate relay connected to said direct contact heater and operatively connected to said ratio relay to nullify the signal transmitted from said water level measurer to said flow control means when the pressure in said direct contact heater is being reduced at more than a predetermined rate.

References Cited in the file of this patent UNITED STATES PATENTS 1,964,773 Smoot July 3, 1934 2,113,559 Abendroth Apr. 12, 1938 2,258,719 Saathofi Oct. 14, 1941