Instrumentation Specialist

Differential inputs provide better common mode rejection and signal-to-noise ratio.

This consist of Instrument location considering the piping drawing given from Piping dept. We identifies the locations of the instrument in the equipment layouts and put the bubbles and elevation and JB nos for the location. Same way depending upon the accessibility we decide the location of JB and marking of it into the instrument location plan is JB location. There is no need to make a different drawing for this.

Control valves must be installed as per direction marking provided by the manufacturers or instruction manuals. Though people tend to generalize, this is often misleading.

In most modern instruments the signal may be programmed to go to either maximum or minimum depending upon end user’s requirement.

A protocol defines a standard method for communications. A fieldbus is a multi dropping arrangement where multiple instruments communicate with special interface hardware using the same pair of wires and in most of the cases draw power from the same pair of wires.

Force balance principle: A controller which generates an output signal by opposing torque. The input force is applied on the input bellows which moves the beam. This crackles nozzle back pressure. The nozzle back pressure is sensed by the balancing bellows which brings the beam to balance. The baffle movement is very less about 0.002 for full scale output.

Advantages:

a. Moving parts are fewer.

b. Baffle movement is negligible

c. Frictional losses are less

The type of switch to use depends on size of the vessel and the location of actuating level. If the vessel is small or actuating level is near the top, top mounted switch can be used. In a large vessel with intermediate actuation points, side mounted switch is used. A top mounted switch needs a stilling well.

Earthing is provided in rack room for purpose of safety. Even if power supplies are kept floating.

In differential tracking system set point changes with change in process value in one direction. E.g. there is trip of OMS in MEG plant on high oxygen flow (with diff tracker). Trip value is say 10% more than normal process value. Now suppose process value is reducing than trip set value will also reduce keeping 10% difference. If process value increased than trip set value will not increase and system will trip when trip set value is reached by process value.

A) Pipe as well as tubes can be used for steam line transmitters. However pipe walls being thicker provide more corrosion allowance.

B) For steam services, with high temp and pressure needs IBR standard. Max. Working pressure is 153 kg/cm2 at 38 degC for 1/2” O.D tube as per design standard. So tube is not suitable.

In numerous applications, it is neither desirable nor practical to expose a temperature sensor directly to a material. Wells are therefore used to protect against damage corresion, arosion, aborsion and high pressure processes. A thermo well is also useful in protecting a sensor from physical damage during handling and normal operations.

Materials used in thermo wells: Stainless steel, Inconel, Monel, Alloy Steel, Hastelloy

The variation in level of buoyancy resulting from a change in liquid level varies the net weight of the displacer increasing or decreasing the load on the torque arm. This change is directly proportional to change in level and specific gravity of the liquid. The resulting torque tube movement varies the angular motion of the rotor in RVDT providing a rotor change proportional to the rotor displacement, which is converted and amplified to a D.C. current.

Bernoulli’s theorem states that the ‘total energy of a liquid flowing from one point to another remains constant’. It is applicable for non-compressible liquids. For different types of liquid flow Bernoulli’s equation changes. There is direct proportion between speed of fluid and its dynamic pressure and its kinetic energy. It can be used in various real life situations like measuring pressure on aircraft wing and calibrating the airspeed indicator. It can also be used to low pressure in the venturi tubes present in carburetor.

Different orifice plates are: 1. Concentric 2. Segmental 3. Eccentric

☛ Concentric: These plates are used for ideal liquid as well as gases and steam service. Concentric holes are present in these plates, thats why it is known as concentric orifice.

☛ Segmental: This plate has hole in the form of segment of the circle. This plate is used for colloidal and sherry flow measurement.

☛ Eccentric: This plate has the eccentric holes. This plate is used in viscous and sherry flow measurement.

When, in some processes, e.g. batch processes, long transient responses are expected during which a sustained deviation is present the controller integral action continuously drives the output to a minimum or maximum value. This phenomenon is called ‘integral saturation of the control unit’. When this condition is met, then this unit is de-saturated.

The commonly used control valves can be defined as follows:

a. Depending on Action:

Depending on action there are two types of control valves 1. Air to close 2. Air to close

b. Depending on body:

Depending on body there are 4 types of control valves

1. Globe valves single or double seated

2. Angle valves

3. Butterfly valves

4. Three way valves

Orifice sizing is done using following information:

1) flow measurement range

2) corresponding diff. Pressure

3) piping dimensions

4) tap locations

5) fluid properties at operating conditions (like density/temp/pressure etc.) Standard formulas are used.

Intrinsic safety is a technique for designing electrical equipment for safe use in locations made hazardous by the presence of flammable gas or vapours in the air. Intrinsically safe circuit is one in which any spark or thermal effect produce either normally or under specified fault conditions is incapable of causing ignition of a specified gas or vapour in air mixture at the most ignited concentration.

This is typically performed in modern programmable instruments by means of measuring actual reference junction temperature using a temperature sensor mounted close to the ref. junction and compensating for the same using appropriate look-up table stored within the instrument’s memory.