Instrumentation and Control Flashcards
What’s instrumentation?
Instrumentation is a branch of physics dealing with the measuring and controlling variables in a process industry. An instrument is a device that measures and regulates the process variables such as flow, pressure, temperature, concentration or level.
What’s a control system?
Control System is an interconnection of components forming a system configuration that will provide a desired response. The key characteristic of control is to interfere, to influence or to modify the process.
What are used of valves?
On/Off service
Pressure / flow rate regulation (Control valve)
Overpressure protection (Relief valve)
Back flow prevention (Check valve)
All valves have a common purpose: to regulate or stop the flow of a fluid.
What are the 3 classifications of valves?
Isolating (Shut-off Valves)
An isolating valve is used only in the closed or fully open position.
Regulating
A regulating valve is used in any position between closed and fully open.
Control
A control valve is power-operated device which changes the fluid flow rate in a process control system.
How can valves be operated?
Manually - by a hand wheel, lever or pedal.
Automatically - driven by changes in air pressure (pneumatic), electric current (solenoid, motor) , or flow (hydraulic).
These changes may act upon a diaphragm or a piston which in turn activates the valve, hence it is called actuator. [An actuator is a component of a machine that is responsible for moving and controlling a mechanism or system, for example by opening a valve. In simple terms, it is a “mover”.]
Why is choosing the correct size of the valve important?
If the size of a valve is not properly selected it can cause many problems including ‘cavitation’ or ‘flashing’ which then reduces the efficiency of the system.
What are the 2 main types of valves?
Linear movement - disc moves up and down. Tight shut-off may be achieved by
tightening down the obturator on a threaded stem i.e gate valves, globe valves, diaphragm valves .
Rotary movement - disc rotates axially. the obturator rotates about an axis at right angles to the
direction of flow. Ball valves and butterfly valves are the two most important rotary valves associated with steam applications.
What are gate valves?
Linear movement
Gate valves are probably the most common valves use in domestic water systems where an uninterrupted flow is required. Used for isolation processes.
The gate, which slides between the seats, is lifted in a direction at right angles to the flow until clear of the flow path. The fact that the gate fully retracts into the bonnet ensures that the pressure drop across the valve is minima.
What are advantages and disadvantages of gate valves?
Adv:
When it is fully opened, the pressure drop across the valve is very small, hence in a process if a minimum pressure drop is required gate valve would be good to use.
Disadv:
Gate valves are used to stop fluids rather than regulate them; precise flow rate control is, therefore, not possible.
The valve can shake violently when high velocity flows or dense liquids are used.
What are globe valves?
Linear movement
Globe valves may have more applications than gate valves as there is a wide variety of configurations available to suit most operations. The movement of fluid through the valve seat is longitudinal to the operating motion of the disk; this means that, the fluid must follow a changing course.
What are advantages and disadvantages of globe valves?
Adv:
A globe valve opens more rapidly than a gate valve as the disc only needs to move a small distance from its seat to allow full flow. This is an advantage when there is frequent operation of the valve.
- can be used instead of bend pipe units
Globe valves are less likely to leak than gate valves, which means that they can be used for higher pressure or higher volume application, such as systems with steam or where fluid loss can be hazardous or costly.
- The increased cost of globe valves over gate valves is therefore offset by the additional safety and reduced chance of fluid loss.
Disadv:
Fluid passes through a restricted opening, and changes direction several times: therefore pressure drop can be large.
If installed in horizontal lines, incomplete drainage can result, and corrosion and contamination can occur.
What’s a diaphragm valve?
What are the 2 types?
Linear movement
The stem of the valve is used to push down a flexible diaphragm, which in turn blocks the path of the fluid. The reinforced diaphragm may be made from natural or synthetic rubber, and may be covered with a resin.
Straight-through type - The bore runs laterally through the body and a wedge shaped diaphragm is used to make the closure.
Weir type - A weir is cast into the body, and when closed, the diaphragm rests on the weir, restricting the flow.
Advantages and disadvantages of diaphragm valve:
Advantages:
The main advantage of a diaphragm valve is the fact that the diaphragm isolates the moving parts of the valve from the process fluid.
They are therefore suitable for handling aggressive fluids and for those containing suspended solids.
In addition, as the bonnet assembly is not exposed to the fluid, it can be made from inexpensive materials such as cast iron, thereby reducing the overall cost. It requires a small maintenance.
Disadvantages:
Their application is however limited by the temperature that the diaphragm can withstand - typically less than 175°C. Maximum pressure allowable is 3.5 bar; as the diaphragm may be damaged.
What’s a needle valve? Adv and disadv:
Linear movement.
They have a relatively small orifice with a long, tapered seat and a needle-shaped plunger. Since it takes many turns of the fine-threaded screw to retract the plunger, precise regulation of the flow rate is possible, although it is generally only capable of relatively low flow rates. Mainly used in the laboratory for pipes less than 2 cm in diameter.
Adv:
They can achieve an extremely accurate flow control for gases and liquids.
They can withstand pressures up to 20 bar.
Disadvantages:
They may create a high pressure drop.
They are extremely expensive if made in stainless steel.
Unlike a ball valve, or valves with a rising stem, it is not easy to tell from examining the handle position whether the valve is open or closed.
What’s a ball valve?
Rotary movement
They have a spherical disc which controls the flow through it. The sphere has a hole, through the middle so that when the hole is in line with both ends of the valve, flow will occur. When the valve is closed, the hole is perpendicular to the ends of the valve, and flow is blocked.
