Copyright 1991 by ASME. The Design and Development of an Electrically Operated Fuel Control Valve for Industrial Gas Turbines

Size: px

Start display at page:

Dwight Johnson
6 years ago
Views:

1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47 St., New York, N.Y (0s The Society shall not be responsible for statements or opinions advanced in papers or in discussion at meetings of the Society or of its Divisions or Sections, or printed in its publications- Discussion is printed only if the paper is published in an ASME Journal. Papers are available from ASME for fifteen months after the meeting. Printed in USA. Copyright 1991 by ASME 91-GT-64 The Design and Development of an Electrically Operated Fuel Control Valve for Industrial Gas Turbines A. G. SALSI Hawker Siddeley Dynamics (Eng) Ltd Welwyn Garden City, AL7 1 LR - UK F. S. BHINDER Hatfield Polytechnic Hatfield, AL10 9AB. UK ABSTRACT Industrial gas turbines operate over a wide range of combinations of loads and speeds. The fuel control valve must be designed to cover the entire range precisely. The design of an electrically operated fuel control valve is described and comparison between the predicted and measured performance characteristics is shown. NOMENCLATURE A = orifice area (m) Al = incremental area C = absolute flow velocity (m/s) Cd = discharge coefficient Cp = specific heat at constant pressure (kj/kg Ii) m = mass flow rate (kg/s) i = molecular weiht P = Pressure (N/m ) R = gas constant (kj/kg 'K) 'I' = temperature (`R1 x = orifice diameter (m) y = spool diameter (m) z = as defined in the text Greek Symbo ls a = angular tolerance at zero flow (rad) 9 = angular displacement of spool (i.e. 0 initial + Alp) I = ratio of specific heats (Cp/Cv) 0 = cut angle (the angle between the inclined surfaces and the spool axis) I = angular tolerance at imum flow ),rad) p = density of the working fluid (kg/m ) $ = angular displacement of spool from imum flow to zero flow 60 = incremental value of t Sub-scr ipts 0 = stagnation conditions 1 = conditions upstream of the orifice 2 = conditions at the minimum flow area INTRODUCTION The fuel system of a gas turbine has often been described as the heart of the engine. The system comprises a fuel reservoir, a pump, a fuel control or flow metering valve, fuel injectors and last but not least the fuel controller. The function of the metering valve is to control the flow of fuel to match the applied load at operational range of speeds of the engine depending on the application. The movement of matering valve is controlled very precisely by the electronic controller. The metered quantity of fuel is supplied to the combustion chamber via the fuel injectors as turbulent.jets in the case of gaseous fuels and as fine sprays in the case of liquid fuels. The industrial gas turbine engines cover a wide range of power outputs from a few hundred kilowatts to nearly a hundred megawatts. A particular gas turbine may operate over a wide range of loads and speeds. The fuel metering or flow control valve must be designed and calibrated to cover the entire range precisely. The design and performance of an electrically operated fuel control valve are described in this paper.. THE DESIGN OF THE VALVE A photograph of the valve assembly and a general arrangement drawing are shown in Fig. 1(a) and 1(b) respectively. The valve proper comprises a spool and a cylindrical block which incorporates two diametrically opposed metering orifices. When the valve is fully open, approximately half of each orifice is covered by the slanting surfaces which are machined on the spool. Thus the total flow area is given by the two equal area half open orifices. The spool is driven electrically by means of a stepping motor. Gov_ e_rni rng_eguations. It can be seen from the photograph and the drawing of the valve shown in Fig. I, that the flow path is quite tortuous. The fuel flows through the two diametrically opposing segments of circular orifices Presented at the International Gas Turbine and Aeroengine Congress and Exposition Orlando, FL June 3-6, 1991

2 Fig. 1. (a) A photograph of the Valve Assembly b. Top Cover Removed and then sharply turns through a 90 degree bend. Hence the flow is extremely complex and it does not lend itself to a reliable analysis. However, in spite of the complex nature of the flow, it has been found that a simple flow model based on the orifice relationships, Owen and Pankhurst (1969), can be used to produce quite good results. But it is very important that the values of the discharge coefficient are selected with utmost care. The method for determining the discharge coefficient as described in BS 1042 (British Standards Institution, 1943) for example is not satisfactory.. The design variables of the valve are: the spool diameter, the diameter of the two diametrically opposing apertures in the valve block and the angle of cut which produces slanting surfaces on the spool. The relationships between the design variables and the performance characteristics are derived in the following. Assuming that the flow through the valve is steady and one dimensional, the energy and continuity equations applied to sections (1) and (2) of a metering orifice, Fig. A.1 - Appendix, would yield the following expression. The derivations of this expression and the geometrical parameters of the valve are given in the Appendix. RT, A = L d A P 01 y P)2/)' P Y (1) 7-1 P ip1 ¼ From the geometry of the valve it can be seen that: z = Z 1 )rad sin 0 12) O O O O O O..S' ro.sawa p12 1 = 2 sin 1 (x/ Ix/2)z cosl sin tg x^2) (3)»l l l New area A' = A - 2 M (4) The mass flow rate of dry air can be calculated as a function of the angular displacement of the spool by substituting A' for A in equation (1). The gas flow rate is given by the following equation: d. Electrical ; ^ Camections 0 0 ^o 00 «o 0 m = m 1/ M (5) gas air air. gas) The results can be calculated in a step by step manner either by hand calculations or by using a computer program. A typical procedure for hand calculations is shown in table 7.. TYPICAL RESULTS Fig. I (b) General arrangement drawing of the valve assembly The procedure described in this paper was used to design a gas valve for a typical industrial application. The design details and predicted and measured performance characteristics are discussed in the following. 2

3 TABLE 1 Relevant Enjine_Performance_Data Power output = 22 MW Specific energy consumption = 9681 kj/kw-hr Compressor delivery pressure = not given Turbine entry temperature = 1067 K Air mass flow rate = kg/s Ambient pressure = Normal atmospheric Ambient temperature = Normal atmospheric Upstream pressure (fuel line) = bar Manifold pressure (fuel) = 20 bar Fuel pipe diameter = 58 mm Pressure loss was calculated for fuel pipe length of 1.52m and with three 90' elbows. Fuel Data Fuel type = Fuel gas Calorific value = kj/m 3 Density ratio (gas/air) = 663 Molecular weight = 19.2 Valve Design Specifications Fuel pressure ratio = : 1 (allowing for system press. loss) 100% fuel gas flow rate = 1.01 kg/s Air flow rate (+8.5% excess) = 1.66 kg/s Valve Dimensions Orifice diameter = 348mm Spool diameter = 39.80mm Angle of cut = 47 0 Angular displacement = 60 0 (including tolerances) The valve area vs angular position graph is shown in Fig. 2. It should be noted that the curve is linear from approximately -25 to +30. Therefore when the orifice is choked, i.e. total to static pressure ratio at the minimum area section is greater than 1.89 for dry air, the flow would be directly proportional to the area. Performance Characteristics The calculated dimensionless mass flow rate vs pressure ratio characteristics for a range of angular positions of the spool are shown in Fig. 3. These curves demonstrate typical metering orifice behavior. The flow rate vs angular position of the spool can be plotted by reading the values of the mass flow parameter at any pressure ratio for the full range of values of the spool angular positions. Such a diagram for the full range of operation of the valve is shown NC a cq^ A. 6> o. o Angular Position (deg) Fie. 2. Valve open area vs the ngular position of the spool in Fie. 4. On this diagram four curves are shown for different pressure ratios including the design pressure ratio of : 1. The normalized flow characteristics for the full movement of the spool from -30'to are shown in Fig. 5. There are two curves on this diagram. These refer to the predicted and measured flows at the design pressure ratio. Agreement between the measured and the predicted results is reasonable, the imum deviation being 10% at the full flow range. The differences may be attributed to the fact that calculations were based on a constant value of the discharge coefficient for the 3

4 14 Angulr Displacement 31 l= 3c :1 :1. 011L.10 on o.09 K.08 ^ 07 a E F.06{ 10 FBI E I a y. 05 L!_ c. r^ 04 ^o W m ^ Pressure Ratio Angular Position (deg) Fl:. 3;.'lass flow parameter vs pressure ratio characteristics for different values of the spool position Fig. 4. Dimensionless Mass Flow vs Angular Position Characteristics metering orifice. However, neither the shape of the aperture produced by the movement of the spool nor the configuration of the up stream and down stream ducts conform to any standards for metering orifices. Therefore the discharge coefficient would not be constant. CONCLUSIONS 1. The design procedure for an electrically operated fuel control valve for industrial gas turbine engines has been presented. The fluid dynamic design is based on the theory of the standard metering orifice. 2. The measured and the predicted mass flow rate characteristics are given. The agreement between these characteristics is reasonable. The imum deviation being 10 %. 3. Small differences between the measured and the predicted results are probably due to the fact that neither the shape of the aperture produced by the movement of the spool nor the configuration of the up stream and the down stream ducts conform to any established standards for metering orifices. 4 At imum rate the flow would be very chaotic thus reducing the discharge coefficient further. Investigations to study the variations of discharge coefficient due with the changes in the shape of the orifice could refine the accuracy of the predictions. C.a a S _ k 50 ti o Angular Position (deg) Fig. 5. Measured and Predicted Mass Flow Rate vs Angular Position Characteristics 4

5 British Standards Institution. Flow Measurement, British Standard Code B.S, 1042; 1943 (UDC ) Owen, E. and Pankhurst, R.C. The Measurement of Air Flow. Pergamon Press 4th Edition, 1966/69 Orifice Flow Equations APPENDIX The cross-section of a D and D/2 British Standard orifice is shown in Fig. 1.A. Assuming one dimensional flow, the mass flow rate at any section would be given by the following expression: p m = C d P p o A C (A.1) 0 Equation A.1 can be written as follows: or; m R To m R To _ = C1/Y C A Po - AP Cd [ P d ( _`Pi o R T o ] l/). RT Fig. A.2. Cross-section of a D and D/2 British Standard Metering Orifice This expression can be written as: or P 2 P2 ( C12 Y - 1 Poz P 1 Il l 2 C T p 01 P ^ f{c) (A.4) 1 For C 1 up to 100 m/s and T 01 down to 300 K, it can be shown quite easily that the values of f(c I ) are close to unity, therefore it is usual to write equation A.3 as follows: Cd P (A.2) 0 0 P Y 7 This expression can be normalized by introducing A 2 - z as follows: Y - 1zP1 pt (A.5) m R to = C A P d A 0 m R Tot, _ A A P ^d A {} The departure of equation A.5 from experimental data due to the simplifying assumptions and real flow effects is generally accounted for by introducing an empirical coefficient C known as the discharge coefficient. 2 Y + 1-, Y Y 2 7P - (A.3) 7-1 Po Po This is the imum flow rate when both apertures are half open. Within the frame work of the afore-mentioned assumptions, equation A.3 would apply to flow at any section of a variable area duct. For isentropic flow through a standard metering orifice P,2 P, 01' therefore: 1) _ P z P 1P2. [ (T01- C12/2C PI Pot PI Pot - ^1 T01 Geometrical Relationships The geometrical quantities of interest are: the orifice diameter; spool diameter; and the valve cut angle i.e. the angle at which the slanting surfaces are cut on the spool. The relationships between these variables are developed in the following: let x = the orifice diameter y = spool diameter,p = angular displacement of spool from imum flow to zero flow a = angular tolerance at zero flow A = angular tolerance at imum flow Using the data given for a particular application the fuel flow can be calculated as follows:

6 _ power output x brake specific fuel consumption mcalorific value of fuel...(a.5a) thermal efficiency n th x isentropic enthalpy drop calorific value of fuel...(a.5b) Since A = A for imum flow, the value of A can be calculated by substituting the!mown values in equation A.4. I I \ I I /%A^: Orifice diameter x = From Fig. A.2, which gives the geometric relationships between the principal variables, it can be seen that: Spool diameter y = x (A.7) sin 2 Fig. A.2. Schematic Diagram Showing the Geometric Relationships Between the Principal Design Variables Spool cut angle A = sin -1 [x/(4.y)j (A.8) Once the orifice diameter has been decided to suit the imum flow and the imum spool displacement is fixed, the spool diameter can be calculated. 6

Discharge Coefficients of Cooling Holes with Radiused and Chamfered Inlets

Discharge Coefficients of Cooling Holes with Radiused and Chamfered Inlets THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47 St., New York, N.Y. 10017 91 -GT-269 r+ The Society shall not be responsible for statements or opinions advanced in papers or in dis- C cussion at