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Valves

 GATE VALVE:
Introduction


Gate valves are used in most of chemical process industries, predominantly in Petroleum industries, because of the attention that it has gained as a good flow isolation valve with very little or negligible leakage. Nearly 70% of the valves in petroleum industries are gate valves. They are suitable for services which require ON/OFF application. Throttling is not preferred with this valve type.
Construction / Working:
                                          Rising stem


The stem rises/lowers with opening/closing the valve.
The stem in closed position is not in contact with the process fluid, hence corrosion issues are NIL in this case.
Requires more space relatively.
All fluids are compatible, usually recommended for high temperature and corrosive fluid application, where its counterpart, non-rising stems is not applicable.
Usually made of cast or forged steel.
                                         Non-rising stem:


·         Stem in this case is static.·         Screw threads of the stem are exposed to process fluid so stem damage/contamination (fouling, scaling) is an issue.·         Requires less space.·         Used only when the process fluid is a clear liquid.·         Usually made of bronze, cast iron, or brass.

GATE / DISC type:
        Wedge type
                                            Solid  wedge:


Does not allow for expansion or contraction of valve body and stem  material with large change in temperature.
Not used of high temperature application.
Can handle any type of fluid.
Available in following MOC:  bronze, cast iron, carbon steel (CS) etc.
                                           Split gate
It allows for expansion and contraction.
Disc has better fit in valve seat over wide range of pressure and temperature.
Appropriate to handle non-condensing gases and liquids.
                                          Flexible gate:






For very high temperature processes, it gives very tight seal on expansion and contraction on the valve seat over wide range of pressure and temperature.
Usually applicable for steam application.
Advantages of Gate valve:
Lowest pressure drop across valve when fully opened
High Cv (flow coefficient)
Low fluid resistance (no obstruction to flow)
Bubble tight shut off / Negligible leakage
High capacity.
Multiple liquids allowed (clean, oil, gas, steam, slurry, corrosive, erosive etc.)
Good sealing performance when fully opened.
Can be operated manually even for larger pipes.
Applicable for wide temperature range and varied line sizes.
Relatively low cost valve.
Disadvantages of Gate valve:
Not suitable for high pressure applications, since gate acts as a diaphragm and can bend or burst under high pressures.
Should be only used for ON/OFF purpose. Throttling causes vibration and loud noise.
Valve disc and seat undergoes a lot of wear & tear, making it necessary to replace them more often.
Possibility of foreign material blocking the gate from sealing.
Maintenance is difficult
More space is required
Long time to open or close valve.

Dimensionless Numbers & their Significance

Nomenclature:

D = diameter of pipe

D_{H =} Hydraulic diameter

L = Length of the pipe

Lch = characteristic length

R = Length through which conduction occurs.

u = mean characteristic velocity of the object relative to the fluid.

Vch = Characteristic velocity

Cp = specific heat capacity at constant pressure.

k = thermal conductivity

μ = dynamic viscosity of the fluid

 = density of fluid.

DAB = mass diffusivity

h = heat transfer coefficient.

g = acceleration due to earths gravity.

t = characteristic time

ν = Kinematic viscosity of fluid.

α = Thermal diffusivity

β = volumetric thermal expansion coefficient ( = 1/T for ideal fluids, T = absolute temperature)

Ts = surface temperature

T_∞ = Bulk Temperature

Significance:

• Ratio of Inertial forces to viscous forces.
• Primarily used to analyse different flow regimes namely Laminar, Turbulent, or both.
• When Viscous forces are dominant its a laminar flow & when Inertial forces are dominant it is a Turbulent flow.

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Significance:

• Depends only on fluid & its properties. It is also ratio of velocity boundary layer to thermal boundary layer
• Pr = small, implies that rate of thermal diffusion (heat) is more than the rate of momentum diffusion (velocity). 
• Also the thickness of thermal boundary layer is much larger than the velocity boundary layer.

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Significance:

• Analogous of Prandtl number in Heat Transfer.
• Used in fluid flows in which there is simultaneous momentum & mass diffusion. 
• It is also ratio of fluid boundary layer to mass transfer boundary layer thickness.
• To find mass transfer coefficient using Sherwood number, we need Schmidt number. 

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Significance:

• Ratio of thermal diffusivity to mass diffusivity.
• Fluid flow with simultaneous Heat & mass transfer by convection.
• It is also ratio of Schmidt number to Prandtl number

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Significance:

• Heat transported by convection to Heat transported by conduction.
• Product of Re & Pr for Pe(HT) & product of Re & SC for Pe(MT)

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Significance:

• It is the ratio of  heat transferred to the fluid to the heat transported by the fluid (ratio of Nusselt number to Peclet number)
• Used to find heat transfer in forced convection flows.
• St(HT) = Nu/(Re.Pr) & St(MT) = Sh/(Re.Sc)

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Significance:

A) Sherwood Number:

• Ratio of Convective to diffusive mass transport. Used in mass transfer operations.
• Analogous of Nusselt number in Heat transfer OR Sherwood number is Nusselt number for mass transfer.

B) Nusselt Number

• Ratio of convective to conductive heat transfer coefficient across the boundary layer.
• Low Nu => conduction is more => Laminar flow 
• High Nu => convection is more=> Turbulent flow.
• It can also be viewed as conduction resistance to convection resistance of the material.
• Free convection: Nu = f(Ra, Pr)
• Forced Convection: Nu = f(Re, Pr)

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Significance:

• Ratio of Buoyancy force to viscous force in natural convection.
• Reynolds number is used in forced convection of  fluid flow, whereas Grashof number is used in natural convection.

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Significance:

• used in unsteady state (transient) heat  transfer conditions.
• ratio of  heat  transfer resistance inside the body to  heat  transfer resistance at the surface of the body. OR ratio of  internal thermal resistance to external thermal resistance .
• Shows the variation of temperature inside the body w.r.t to time.
• Bi < 0.1 => heat transfer resistance inside the body is very low => inside the bodyconduction takes place faster compared to convection at the surface. => no temperature gradient inside the body (uniformity in temperature) vice versa implies that Temperature is not uniform throughout hte material volume.

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Significance:

• It shows the presence & strength of convection in a fluid body.
• Heat transfer by Conduction within fluid < Critical value for that fluid < Heat transfer by convection. (consequences of Ra values)
• Product of Gr.Pr

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Significance:

• Characterizes laminar flow in a conduit OR transfer of heat by streamline fluid flow in a pipe
• In case of  mass transfer, Pr is replaced by Sc.

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Significance:

• Ratio of rate of heat conduction to the rate of heat storage.
• Used along with Biot number to solve transient state heat transfer problems.
• For mass transfer by diffusion, Fourier number for MT is used.
• It can also be understood as current time to the time taken to reach steady state.