Valve
a device that regulates, controls, or directs the flow.
The throttling is an isenthalpic process, which is very imporatant for expansion calculation. It means that if an expansion valve functions very quickly, the pipeline without isolation can have ice on it. Since the ambient air has no time to give back the heat. In order to prevent ice formation, we may lower the expansion velocity through the use of multiple orifices or heat isolation.
To compare different valves, kv-values and opening percent should be compared.
Every controlling valve has a lock valve (Absperrarmatur) before the controlling valve. If the controlling valve does not function well, the hand fitting will be closed or opened to help the controlling.
Calculation of leakage rate:
Leakage is like an orifice. You have somewhere a velociy higher than sonic sound. There is a norm for the size of the hole. According to the API 521 (4.4.9.3.2), the hole area should be estimated as 1/10 of the pipeline diameter for the check valve.
Mole number after leakage = mole number before the leakage + (leakage rate * time* pressure before the leakage/(RT))
The relation between Kv value and temperature
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. For the equal percentages, the slope by higher stem openings is higher than the linear. It means that a small change in the stem openings can result in a higher Kv-Value and flow rate.
Actuator: for opening and closing a valve using gas pressure , liquid pressure (hydraulic) or electricity through the linear or rotary motion .
- Manual
- Pneumatic
- 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)
- Spring
- Solenoid
- Pressure regulator (Druckminderer):
Two springs, A and B balance each other with an adjustable orifice located between. As you increase spring A pressure by screwing in the adjusting screw, the orifice opens letting water into the outlet. As the pressure build-up in the outlet, this pushes against the diaphragm which added to the force of spring B, closing the orifice.
If a valve is defect, the actuator and valve should be check sepearately to see, which part should be exchanged.
An Electric Initiator (EI) converts mechanical energy to electrical signal. It is a black box on the valve. Through the mechanical movement of the actuator and valve, there will be a voltage, which is sent as a signal to the control room. The initiator is regularly defect, if the valve is often opened and closed. Sometimes, the signal is weak. Therefore, it can not reach the control room. Sometimes, the valve is mechanically defect and it can bring the initiator the force, since it does not move.
If the safe position of the valve is defect, the valve should be exchanged.
If a valve is too big, there is a problem due to the high gain. To prevent it, we can close the hand lock valve (Absperrarmatur) before the controlling valve. The desired opening can give us the Kv-value. Q rate is known. Delta P of the valve can be calculated. Considering delta P of the system and charcteristic curve of the lock valve before the controlling valve, the closing percent of valve can be estimated.
Every valve has a sealing, where the spindel leaves the housing and goes to the drive.
Die Regelstrecke muss schnell sein, Das ist der geschwindigkeitbestimmte Schritt. Antrieb ist schnell und Regelstrecke ist langsam.
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
Compared with a gate valve or ball valve, the globe valve has considerably higher pressure loss in the fully open position. This is because the flow of fluid changes direction as it goes through the valve.
3. Check valve: to convey fluid in one direction. Check valve can be Rückschlagklappe or other valves.
If we have no stable dosing due to having a very big controlling valve, we can control it with the check valve prior to the controlling valve. The check valve should’nt be a Rückschlagklappe, but a Regelklappe.
- Reducing the pressure through:
- Closing the lock valve manually to reach more opening of the check valve
- Using a Druckminderer
- Using an orifice
- Increasing the temperatur prior to the lock valve: The higher temperature in gas, the higher the viscosity will be and the higher the friction will be. For the liquid, it will be vice versa.
4. Plug valve:
5. Rotary plug valve (drehkegelventil)is mainly used for applications with high solid contents resulting in cavitation and erosion. One of the main advantages of the rotary plug valve is its free passage. Due to the flow restrictor which moves crossways to the flow, the media current does not have to be diverted through the housing wall.
5. Ball valve: rotational motion, shuttoff application for gas.
The pressure loss through the ball valve at fully opening position can be ignored.It means that the Kv value would be the same as the volumetric flow rate through the valve.
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): It has a diaphragm and spring. The pressure would be controlled by the spring force. It provides a constant downstream pressure while delivering the required flow. If the downstream demand for flow decreases, downstream pressure increases. The force against the weight becomes more. Unbalance causes the restricting element to move up to pass flow or lockup.
14. Three-way valve: it has a switch (Weiche), which regulates the way of stream.
15. Relief valve: blows off, while the valve opens.
Joule-Thomson Effect
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)
At room temperature, all gases except hydrogen, helium, and neon cool upon expansion by the Joule–Thomson process. For ideal gases, this coefficient is zero.
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.
General Control Valve Rules of Thumb
- Design tolerance. Many use the greater of the following:
Qsizing = 1.3 Qnormal
Qsizing = 1.1 Qmaximum
2. Type of trim. Use equal percentage whenever there is a large design uncertainty or wide rangeability is desired. Use linear for small uncertainty cases.
3. Limit max/min flow to about 10 for equal per- centage trim and 5 for linear. Equal percentage trim usually requires one larger nominal body size than linear.
4. For good control where possible, make the control valve take 50%-60% of the system flowing head loss. By having a large pressure loss span, the flow rate can be controlled.
5. For saturated steam keep control valve outlet velocity below 0.25 mach.
6. Keep valve inlet velocity below 300ft/sec (91 m/s) for 2" and smaller, and 200 ft/sec (60 m/s) for larger sizes.
Flow coefficient value (Kv or Cv)
Across a control valve the fluid is accelerated to some maximum velocity. At this point the pressure reduces to its lowest value. If this pressure is lower than the liquid’s vapor pressure, flashing will produce bubbles or cavities of vapor. The pressure will rise or “recover” downstream of the lowest pressure point. If the pressure rises to above the vapor pressure. the bubbles or cavities collapse. This causes noise, vibration, and physical damage. When there is a choice, design for no flashing. When there is no choice. locate the valve to flash into a vessel if possible. If flashing or cavitation cannot be avoided, select hardware that can withstand these severe conditions. The downstream line will have to be sized for two phase flow. It is suggested to use a long conical adaptor from the control valve to the downstream line.
When sizing liquid control valves first use
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=0.865 Cv
Kv formula for gas:
For compressible fluids one must be careful that when sonic or “choking” velocity is reached, further decreases in downstream pressure do not produce additional flow. This occurs at an upstream to downstream absolute pres- sure ratio of about 2 : 1.
If pressure drop is high enough to exceed the critical ratio, sonic velocity will be reached.
I. Subcritical: P2>0,5*P1(Backpressure opposing the desired flow)
II. Supercritical
If the temperature goes up, the density goes down und the volumetric flowrate will be higher for the same mass flow rate. That means a higher kv-value. The higher volumetric flowrate causes a higher pressure loss for the same surface area.
Choked flow is a limiting condition where the mass flow will not increase with a further d
ecrease 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.
Cavitation destroys the surface locally, which causes erosion, while flashing causes erosion due to high velocity.
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
For ideal gases:
Speed of sound in water: 1.48 m/s
Speed of sound in solids: 5.2 m/s
Speed of sound depends on the density and stiffness:
The more the stiffness, the higher the speed of sound
The lower the density, the higher the speed of sound
For instance, iron has a higher stiffness (bulk modulus in pascal) than air, but air has a lower density than iron. It results in higher speed of sound through the iron.
Stiffness relates to how a component bends under load while still returning to its original shape once the load is removed.
Druckstoß (Water Hammer) for liquids:
a rapid change in the flow velocity, in which the closing time of valve plays an important role. The water hammer will be calculated only for switch valves having a closure time fewer than 1 second.
The calculation of the pressure to see, whether all fittings can stand the pressure:
Through the formula F= m*a, the calculation can be carried out. The pressure rise through the sudden close can be calculated as the belows: (Joukowski Equation)
∆ P= c v ρ
where ρ is the fluid mass density and c is the speed of sound
When a body is subjected to a system of forces, then there will be a change in the volume of the body due to the stress acts on the body. Bulk modulus, K, is the measure of resistibility to the external forces acting on the body. whereas Young’s modulus , E, is stiffness in the body.
Wave reflection time:
In water hammer analysis, a time constant that is often used describes the progression of the wave from its inception to the secondary barrier and then back again. It takes the form of Tc = 2L/a (where L is the pipe length and a is the velocity of the wave, which is the speed of sound).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 restriction orifice coming after quick closing valve (Schnellschlussventil)ensures that the flow is not excessive to overload the flare. The more the thickness of the orifice, a higher pressure loss can be considered. If the restriction orifice is too big, we may add another restriction orifice to prevent the overload of flare.
The restricting orifice has a critial pressure loss, which results a choked flow, while the differential pressure sensor has only an acting pressure loss (wirkende Druckverlust) and permanent pressure loss (bleibende Druckverlust). The differential pressure sensor has no critital pressure loss, but a measuring span of an acting pressure loss, which is dependent on the fluid, gas or liquid.
The acting pressure loss for flow measurement should be around 250 mbar for liquids and 125 mbar for gas phas. Due to the compressibiliy of gas, it should be much lower than for liquid.
The flow rate can be calculated by square root extractor coming from Bernoulli equation..
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
With Orifice Plate Flow Meters, there must be a laminar flow to the orifice plate for minimal error in metering. The piping fitting(s) like bend or valve will have influence on the fluid behavior and the required length of straight pipe prior to the meter to deliver laminar flow: 10–15 * Diameter of pipeline.
Wenn es einen Abzweig gibt, der nicht durchströmt war, kann man das wie gerades Rohrstück nehmen. Das sagt PTB (physikalisch technisch Bundesanstalt)
2. Fluid dynamic (vortex shedding): Vortex flow meters are also insensitive to temperature, pressure, and viscosity. The disadvantage of a vortex flow meter is that there is a low to medium pressure drop due to the obstruction in the flow path. The basic principle for a vortex flow meter is that a barrier is placed in a moving stream. The basic principle for a vortex flow meter is that a barrier is placed in a moving stream. As the flow goes around the object it alternates creating vortices (swirls of media) from the top or bottom of the object. The swirls are created from the increase in pressure and decrease in velocity on one side of the object and a decrease in pressure and increase in velocity on the other side.
3. Ultrasonic
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
Calculation of the process safety time for the valves:
Every valve has a process safety time, which is the time period between a failure occurring in the process or the basic process control system with the potential to give rise to a hazardous event.
The process safety time is calculated by the (maximum reachable value- switch value)/maximum rate of change. The process safety time can always be assumed as 30 seconds.
Example: If you have a vessel with a switch level at 90%, you can calculate the volume difference between 100 and 90% and divide it into the feed rate.
If there is an automatic switch, the operation time is zero and only the runtime of the actor (Laufzeit Aktor) should be considered. That is the longest action, in which the electrical energy is converted into mechanical energy (air pressure).
Every valve has a fail safe position. If a valve is closed in its fail safe position, the valve takes more time for opening as closing. Hence, the runtime of actor will be estimated only for opening. This time would be around 3 or 4 seconds. Over the years, it will be longer due to a leakage of air.
