a device that regulates, controls, or directs the flow.
The throttling is an isenthalpic process, which is very imporatant for expansion calculation.
Characteristic Curve of Valve
1. Installed Characteristic Curve: plots the valve open percent versus the flow through the valve, an dependends on the conditions specific to the system
2. Inherent Characteristic Curve: provided by manufaturer, represents the relationship between valve flow capacity and valve opening when there are no system effects involved.
The majority of control applications for systems with a significant amount of pipe and fittings are with equal-percentage.
Actuator: for opening and closing a valve using gas pressure , liquid pressure (hydraulic) or electricity through the linear or rotary motion .
- Hydraulic, a fluid-operated device: A hydraulic valve properly directs the flow of a liquid medium, usually oil, through your hydraulic system. This type of valve is used in the sealing of the compressor to control the flow rate of oil according to the required pressure.
- Electric (motor)
Different types of valve
you may underestand from the colours or the wheel, whether the valve is open or closed. When the wheel is on the top level, the valve is open.
- Gate Valve (Absperrschieber): linear motion, shutoff application. At high pressures, friction can become a problem. The seating load of the larger gate valves can become so high at high fluid pressures that friction between the seating’s can make it difficult to raise the disc from the closed position.
I. Bypass: to reduce the pressure before operating the gate valve itself.
II. An increase of pressure on the discharge side by a high pressure vessel
A gate valve has two lines, which are the indicators of this type.
2. Globe valve (Durchgangsventil): a linear motion, good for regulating
3. Check valve: to convey fluid in one direction. Check valve can be Rückschlagklappe or other valves.
4. Plug valve
5. Ball valve: rotational motion, shuttoff application for gas
6. Niddle valve: linear motion, for controlling low flow
7. Pinch valve
8. Pressure relief valve
9. Butterfly valve: is similar to ball valve
10. Solenoid valve
11. Angle valve: for transferring compressed fuels, water companies to prevent backflow of conatminants
12. Rotary valve (Zellradschleuse): for transporting solids. This type of valve is not tight. For transporting solid, gas or liquid can be used. Gas would be recommend for the better separation.
- Innere Undichte: Trotz geschlossener Armatur gibt es einen Leckstrom von der Hochdruckseite zur Niederdruckseite.
- Undichte von aussen nach innen: wenn das System unterdruck ist. Es kann Ex-Schut relevant sein, wenn brennbare Medien im Vakuum sind.
Switch valve (Schaltventil) versus control valve Regelventil:
Switch valve contains a switch, while control valve does not have any.
13. Druckminderer (Pressure regulator):
describes the temperature change of a real gas or liquid (as differentiated from an ideal gas) when it is forced through a valve while keeping it insulated so that no heat is exchanged with the environment (adiabatic process)
The temperature of this point, the Joule–Thomson inversion temperature, depends on the pressure of the gas before expansion.
The maximum pressure drop in a system including pump, vessel and elevation: pressure in pumpe (at 0 m3/hr) + P set of safety valve of vessel + elevation in vessel and before the pump
To have an estimate of pressure drop in valve:
For existing valve
Required pressure drop in valev: Inlet pressure from battery limits- outlet pressurebattery limits-pressure drop in piping-pressure drop in heat exchanger
For new valve: using the minimum allowable pressure drop from the data sheet of valve, provided by the manufacturer.
Flow coefficient value (Kv or Cv)
Kvs: express the amount of flow at fully open position and pressure differential of 1 bar.
Kv: express the amount of flow at a given position and pressure differential of 1 bar.
The Kv formula is based on the hydrodynamic law from Bernoulli’s equation: double the flow: pressure drop four times
Kv formula for liquid:
Kv formula for gas:
I. Subcritical: P2>0,5*P1(Backpressure opposing the desired flow)
is a limiting condition where the mass flow will not increase with a further decrease in the downstream pressure environment for a fixed upstream pressure and temperature. The velocity of choked flow is equal to the sound velocity.
Choked flow in liquids
venturi effect acting on the liquid flow through the restriction causes a decrease of the liquid pressure beyond the restriction to below that of the liquid’s vapor pressure at the prevailing liquid temperature.
Choked flow in gas
occurs when the downstream pressure falls below the critical pressure (in which a liquid substance can coexist with its vapour).
The volumetric flowrate: sound velocity * discharge hole cross-sectional area
Speed of sound: distance travelled per unit time by sound wave depends strongly on the temperature.
Speed of sound in air: 343 m/s
Speed of sound in water: 1.48 m/s
Speed of sound in solids: 5.2 m/s
For ideal gases:
Different Way of Measuring Flowrates
1. Orifice Plate
for measuring flow rate, for reducing pressure or for restricting flow (in the latter two cases it is often called a restriction plate), is located before the valve, since the flow regime after valve is inconsistent.
The direction of orifice should be checked before start-up.
Through the continuity and Bernoulli equation , the flowrate formula through the orifice can be calculated by assuming steady-state, incompressible (constant fluid density), inviscid and laminar flow.
- The velocity of the fluid particle in the centre of a pipe is maximum and due to the friction, it gradually decreases towards the pipe walls. Thus while using Bernoulli’s equation, only the mean velocity of the liquid should be taken into account.
- There are always some external forces acting on the liquid, which affects the flow of friction. Thus while using Bernoulli’s equation all such external forces are neglected which has not happened in actual practice. If some energy is supplied to or extracted from the flow, the same should also take into account.
- In turbulent flow, some kinetic energy is converted into heat energy and in viscous flow, some energy is lost due to shear forces. Thus while using Bernoulli’s equation all such losses should be neglected, which has not happened in actual practice.
- If the fluid is flowing by a curved path, the energy due to centrifugal forces must also be taken into account.
The microscopic Bernoulli equation is obtained by dotting the Navier Stokes Eqn. (microscopic momentum balance equation) with the fluid velocity. This is usually referred to as the mechanical energy balance equation containing energy from gravity, viscosity, and differential pressure.
Force: momentum change/time
I. Linear Momentum for a linear motion
II. Angular Momentum for a rotational motion
- a point object L (vector)= r * P (vector)
- an extended object : the mass is distributed about the entire object
Inertia: an object’s amount of resistance to change in velocity
If there is a bend in piping, the required length to the orifice for a consistent flow regime: 10–15 * Diameter of pipeline
2. Fluid dynamic (vortex shedding): vortex: a region, in which the flow revolves around an axis line
4. Mass flowmeter (Coriolis force)
For rotational objects:
I. Rotational force: acts outward
II. Coriolis force: acts perpendicular
5. Thermal energy with a heating element
6. Pitottube (Staurohr) can be used for the flow measurement
Stagnation pressure= static pressure + dynamic pressure