1 OBJECTIVE

PARAMETRIC STUDY OF GATE VALVE by changing lift from 10mm to 80mm. Also calculate Cv & Kv.

2 INTODUCTION

Gate valve is an important device used in industrial application where flow of fluid like oil & gasses are involved. It has its application in automotive, marine, pharmaceuticals, & manufacturing industry also. It can be installed above-ground & as well as under-ground. The function of gate valve is to pass the fluid either in fully open condition or hinder the flow in fully closed condition. However, throttling can also be achieved with the help of gate valve but it is not recommended. Throttling causes vibration inside the system which causes erosion of seat & disc so this device is not suitable for application like mass flow control of fluid. This vibration can also cause leakage in the system. Their performance is excellent if used as a shutoff valve. Design of gate valve is done in such a way that it ensures minimum pressure drop. This can be achieved by designing the gate valve in such a way that the diameter of valve in fully opened condition must be equal to diameter of pipe. These valves are manually controlled. There is a disc mounted over spindle. Disc is rotated to open or close the flow path. It is a time taking process to close or open the path because valve has to turn multiple times to go from one end to another extreme end. This makes the operation slow & helps in preventing water hammering effects. These valves are also helpful in creating a by pass channel for fluid passage. By pass channel is helpful in balancing differential pressure of pipeline & lowering torque requirement of the valve. It is also helpful in avoiding stagnation points inside flow field if main valve is closed & by opening by pass valve. By pass valve also helps in delayed filling of pipelines. Below diagram shows different variety of valves having different types of disc for stopping the flow passage.

They are also used in applications that involve viscous liquids such as heavy oils, light grease, varnish, molasses, honey, cream and other non-flammable viscous liquids. They are available in large sizes to better handle thick flow. However, gate valves do have low-pressure limitations.

Now we will discuss specification of gate valve. Gate valve is specified by number of parameters like valve size, pressure reading, body material, metal seated type, type of actuation, method of connection, & media description (fluid type). We have to define all above parameters while designing any gate valve for proper function under operating condition of temperature, pressure & all other flow parameters. Valve size represents the size of opening for fluid passage by the manufacturer. Pressure rating is a very important parameter we should take care of while installing of valves. If fluid pressure will increase the pressure rating, it may cause bursting or leakages in pipes. So, we should choose our valve carefully after analysing the flow pressure. Body material helps in resisting all type of impact created while working conditions. Gate valves are available in form of Aluminium, Brass, Bronze, cast iron, copper, ductile iron, steel, stainless steel etc as a body material. Metal seated helps in operating under high pressure & high temperature operating conditions. Seats are manufactured with material like carbon & stainless steel & it also depends on type of applications. Actuation mechanism helps in controlling the opening & closing of valve. Variety of actuation mechanism are available like manual, hydraulic, pneumatic, electrical, thermal etc. It helps in ease of operation. Method of connection refers that how the inlet & outlet pipes are connected to each other. Main focus of connection is to achieve a seal prof connection in order to avoid leakages. Some of the connection types available are threaded, compression fitting, bolt flange, clamp flange, Butt weld, metal face seal etc. Every connection type has its own advantages & disadvantages & its selection depends on type of industrial application. The last but not the least is media description. Media description refers to the fluid which is to be used in industry. Some of fluid are air, compressed air, breathable air, fresh & dirty water (hot & cold), untreated water (contains contamination), gas, diesel fuel, LPG, natural gas, powder/solids, steam etc. Overall gate valve has wide scope in industrial application.

There is some advantage of gate valve. It has a very simple closing feature & can be used as bidirectional. We have to not be very specific while installation that which side should be installed as inlet & which as an outlet. It also provides minimum pressure loss due to laminar flow. At the same time there are some limitations of using gate valve. It can’t be closed & open quickly & frequently. It requires large space for installation & maintenance purpose. While gate valve is in close position, it causes collision, abrasion & deformation in seating due to vibration. It can cause surface damage & will affect the performance of valve. Sometimes temperature fluctuation is high & irregular due to high load in pipe at valve end. This irregular fluctuation may cause leakages in gate valve. It is also difficult of maintaining seating surfaces.

3 GEOMETRY & MESHING

3.1 GEOMETRY

Following figure clearly shows the geometry of gate valve with proper nomenclature. This is a manual actuator type valve. Every part of valve has shown clearly in following figure.

Above geometry shows a wedge instead of disc. Our cad model has a disc type structure as a stopper for fluid passage. Below figure shows the cad model that has been used for simulation.

Figure a show the entire cad model of gate valve from outside & figure b shows interior components of gate valve. This disc can be lowered or raised as per requirement to control mass flow rate at outlet. It has been already discussed that it can be used as a bidirectional device it means that we can attach inlet & outlet pipes at any of two opening available.

Above figure shows gate valve installed in pipe with fully closed (0mm raise of disc) & fully open (110mm raise of disc). We can give any amount of raise of disc for simulation purpose but any intermediate lift of gate valve is not recommended for practical industrial application.

3.2 MESHING

Automatic method of meshing has been done with element size of 92.89 mm. Automatic method gives pretty fine mesh for further flow analysis. It resulted in 135366 number of element & 27073 number of nodes. Figure below shows the meshing structure.

4 PHYSICS & SETUP

4.1 BOUNDARY CONDITIONS

Simulation was done in presence of gravity for 250 iterations. Water (liquid) was chosen as a working fluid for the setup with standard fluent water property. The inlet boundary condition was chosen as pressure inlet & gauge pressure at inlet was maintained at 10 Pascal. The outlet boundary condition was maintained at zero-gauge pressure. The initial lift was 10 mm & it was increased up to 80 mm lift as shown in figure below

A lift of 0 mm corresponds to closed position of valve & lift of 100 mm corresponds to fully open position of valve as shown in above figure.

4.2 VELOCITY-PRESSURE COUPLING

Coupled scheme was used for velocity pressure coupling method. The pressure-based solver in Fluent offers segregated approach & non-coupled approach to solve the fluid flow domain. There are some advantages of using coupled approach over segregated approach. This scheme provides robust & efficient solution when compared to segregated method of pressure velocity coupling. This pressure-based coupled algorithm offers an alternative to the density-based and pressure-based segregated algorithm with SIMPLE-type pressure-velocity coupling. Segregated based algorithm does separate solution for momentum equation & pressure correction equation but coupled technique has capacity to solve those equation simultaneously & as a result convergence of solution occurs faster in coupled based algorithm. Segregated algorithm is a semi explicit type & we can achieve fully implicit coupling through an implicit discretization of pressure gradient terms in the momentum equations.

4.3 TURBULENCE MODELLING

Realizable K-epsilon with scalable wall function was chosen as turbulence modelling. Realizable model satisfies some mathematical constraints based on Reynolds stress which are consistent with turbulent flow. This formulation contains a new term for turbulent viscosity & equation for the dissipation rate, , has been derived from an exact equation for the transport of the mean-square vorticity fluctuation. Realizable k-epsilon model predicts more accurate results for spreading rate of both planar and round jets. It has been found that this model is best suitable for rotational flow field, presence of adverse pressure gradients, separation, and recirculation. The flow near wall region has been found as a laminar flow even if turbulent flow is dominating in fluid domain. The main reason of using wall function is absence of turbulent flow near wall & k-epsilon turbulent model will not predict correct result near wall region. Wall function helps in capturing near wall effects properly & comes into picture only if 2 equation model (k-epsilon) or 7-equation model (Reynolds stress turbulence model). The purpose of scalable wall functions is to force the usage of the log law in conjunction with the standard wall functions approach. It also ensures that Y⁺ value is greater than 11.126 irrespective of near wall grid refinement. This value helps in marking the intersection of linear & logarithmic velocity profiles. If the value of Y⁺ is greater than 11.126, then scalable wall function & standard wall function predicts identical results. It also takes care of modelling of laminar & buffer region by shifting near wall mesh point to Y⁺ = 11.126 regardless of the thing that how close it is to the wall.

5 RESULT & DISSCUSSION

5.1 RESIDUE-PLOT

Figure below shows the residue plot. The lift has been raised continuously from 10 mm to 80mm & corresponding plot has been observed

5.2 OUTLET MASS FLUX-PLOT

Above plot shows the plot of mass flux. The lowest value of lift was kept at 10mm & highest value was kept at 100mm.

5.3 PARAMETRIC STUDY

The simulation was done initially for 10mm of lift only & we have created a parameter for lift of disc. Later on, we created an output parameter for mass flux. Finally, 10 design points has been created & updated. Table below shows value of mass flux corresponding to the lift of disc. These values were noted at the end of 250 iteration of simulation

The result shown in above table was expected. As we increase lift the area through which fluid will pass keeps on increasing & at a constant flow rate the mass flux will increase.

Above plot shows the trend of mass-flux with respect to corresponding lift of disc. This has been plotted using MATLAB.

EXPLANATION FOR UNEXPECTED RESULT AT 60 mm OF DISC LIFT

In section 5.2 mass flux has been plotted. It can be seen that there is a huge fluctuation in mass flux at output. At every new iteration there will be a huge difference in mass flux at output at a lift of 60mm. It means that solution has not converged for 60mm of lift. We can’t take this value as a final value at this moment we have to do few more iteration.

Remaining all mass flux plot are either having no fluctuation or very minimal fluctuation. So, we can treat those values as final value of mass flux at output.

RE-SIMULATION HAS BEEN DONE FOR 2600 ITERATION. IT HAS BEEN FOUND THAT MASS FLUX AT OUTPUT HAS BEEN TILL KEEPS FLUCTUATING.

Figure below shows mass flux plot for 2600 iteration.

5.4 VELOCITY CONTOUR AT DIFFERENT LIFT

Following plot shows the fluid flow from inlet to outlet.

Right side is INLET & left side is OUTLET as shown below. Same assignment has been done in above plot.

5.5 FLOW COEFFICIENT AND FLOW FACTOR

Flow coefficient- Cv, and Flow factor- Kv, are values used to specify the hydraulic capacity of a control valve.

5.5.1 FLOW FACTOR-K_V

Flow factor is the metric equivalent of flow coefficient and is defined as: the flow of water in metres cubed per hour (m³/hr) at a pressure drop of bar.

Kv= Q*(dP/Sg)^0.5

Where Sg for water is 1 & dP =10 Pascal (converted into bar in matlab code), Q is in m^3/s

5.2 FLOW COEFFICIENT-C_V

Flow coefficient is an imperial measurement and is defined as: the flow of water through a valve at 60°F in US gallons/minute (gpm) at a pressure drop of 1lb/in².

Cv = 1.156 * Kv

When flow goes through a valve or any other restricting device it loses some energy. The flow coefficient is a designing factor which relates head drop (Δh) or pressure drop (ΔP) across the valve with the flow rate (Q).

The flow coefficient of a device is a relative measure of its efficiency at allowing fluid flow. It describes the relationship between the pressure drop across an orifice valve or other assembly and the correspondingflow rate.

The use of the flow coefficient offers a standard method of comparing valve capacities and sizing valves for specific applications that is widely accepted by industry.

FOLLOWING MATLAB CODE HAS BEEN USED TO CALCULTE FLOW FACTOR & FLOW COEFFICIENT. THIS CODE WILL ALSO PLOT ALL RELEVANT GRAPH.

clc;
clear all;
n=7;
x=[10 20 30 40 50 70 80];
y=[0.23919 0.35924 0.46892 0.5642 0.62939 0.7671 1.0697];
plot(x,y,'c*')
hold on
plot(x,y,'c')
hold off
xlim([10.0 80.0])
ylim([0.200 1.100])
legend('mass flux at 10 gauge inlet pressure')
title('LIFT V/S mass flux rate at OUTLET')
xlabel('lift of disc in mm')
ylabel('mass flux rate in Kg/s')

grid on

xlim([10.0 80.0])
ylim([0.200 1.100])

% FLOW COEFFICIENT & FLOW FACTOR CALCULATION
for i=1:n
    Q(i)=y(i)*3.6;     % converted Kg/s into m^3/h
end
dP=10/100000;          % PRESSURE DROP IN BAR
Sg=1;
B=dP/Sg;
A=1/B;
for i=1:n
    q=Q(i);
    K=q*sqrt(A);
    kv(i)=K;             % flow_factor_in__ = [m^{3}.h^{-1}.bar^{-0.5}]
    Cv(i)=1.156*kv(i);   % flow_coefficient
end
plot(x,kv,'r')               % ploting_flow_factor
hold on
plot(x,Cv,'c')               % ploting_flow_coefficient
hold on
plot(x,kv,'r*')
hold on
plot(x,Cv,'c*') 

xlim([10.0 80.0])
ylim([80 450])
grid on
legend({'Kv-FLOW FACTOR','Cv-FLOW COEFFICIENT'})
colorbar
title('HYDRAULIC CAPICITY OF GATE VALVE')
xlabel('DISC LIFT')
ylabel('Cv & Kv')



% CREATE TABLE
design_point=[1;2;3;4;5;6;7];
DISC_lift = [x(1);x(2);x(3);x(4);x(5);x(6);x(7)];
mass_flux = [Q(1);Q(2);Q(3);Q(4);Q(5);Q(6);Q(7)];
flow_factor = [kv(1);kv(2);kv(3);kv(4);kv(5);kv(6);kv(7)];
flow_coefficient = [Cv(1);Cv(2);Cv(3);Cv(4);Cv(5);Cv(6);Cv(7)];
OUTPUT_TABLE = table(DISC_lift,mass_flux,flow_factor,flow_coefficient)

6 CONCLUSION

6.1 MASS FLUX

It has been seen that mass flux is continuously increasing if we keep increasing the disc lift. We have already discussed that it is NOT RECOMMENDED that to use gate valve to REGULATE flow because it causes vibration of setup. It may be one of the main reasons behind the HUGE FLUCTUATION in mass flux at 60mm lift of disc. This has been shown clearly in figure below.

6.2 FLOW COEFFICIENT

Number of US. GALLONS PER MINUTE of 60F water that will flow through a FULLY OPEN VALVE with a 1 PSI DROP across it.

Figure below shows plot of Cv & Kv with increasing valve size.

Following Table shows the calculated values of flow factor & flow coefficient for gate valve. These values has been calculated with the help of matlab. Trendline shows an increase in both parameter as we increases the lift of disc.

	DISC_lift in mm	mass_flux in m^3/s	flow_factor_in [m^{3}.h^{-1}.bar^{-0.5}]	flow_coefficient
1	10	0.8611	86.1084	99.5413
2	20	1.2933	129.3264	149.5013
3	30	1.6881	168.8112	195.1457
4	40	2.0311	203.1120	234.7975
5	50	2.2658	226.5804	261.9269
6	70	2.7616	276.1560	319.2363
7	80	3.8509	385.0920	445.1664

The value of Cv & Kv at design point 7 (fully opened condition of valve) is considered as design value of gate valve. This value helps to calculate the mass flux of gate valve.