Advantages of Field Calibration vs. Bench Calibration

A Bench Calibration is carried out in the shop at the bench with power supplied from an external source, if necessary. Bench calibrations might be performed upon receipt of new instruments  prior to installation. This provides assurance the devices is received undamaged. This also allows configuration and calibration in a more good environment.

Various companies perform periodic calibrations on the bench. In this case the process instrument is removed from service, disconnected and taken to the shop for calibration. In some situations, a spare is installed in its place so the process downtime is minimized. For example, critical flow sensors might be sent out a specialized flow calibration facility. To prevent shutting the process down for several weeks, a replacement flow sensor would be installed.

Field  calibrations are carried out "in-site", or in place, as installed. The instrument being calibrated isn't removed from the put in location. Field calibrations may be carried out after set up to ensure proper connections and configurations. Periodic calibrations are more likely to be carried out in the field. 

Field calibrations are performed in the location in which the instrument operates. If the instrument is installed in a harsh location it is calibrated for that location. If the instrument is removed for a bench calibration and then returned, some error might be introduced due to the ambient conditions and orientation.

Advantages of Bench Calibration

• Removed, cleaned, inspected
• Better work environment
• Fixed calibration setup and utilities (electrical, air, vacuum) available

Advantages of Field Calibration

• May save time
• May identify and allow troubleshooting of installation problems
• Performed in actual ambient environment

Read More

Coriolis Mass Flowmeter

A mass flowmeter measures flow rate in weight per unit time rather than volume. This measurement compensates for temperature and pressure changes. Fluid moving through a vibrating tube is forced to accelerate as it moves toward the point of peak amplitude of vibration. The fluid decelerate as it moves away from the point of peak amplitude. The acceleration and deceleration cause twisting forces on the flowtube which are proportional to the mass flow.

Sample: SERIES CM : Coriolis Mass Flow Meter

Read More

Control Valve Calibration

As in positioner calibration, a pressure signal is applied to the actuator and the resulting valve position is recorded. This may be carried out with the positioner calibration, if applicable. It may also be performed in conjunction of I/P calibration as described. Just remember to make sure the system is in safe condition if performing the calibration in the field. Also remember that you need to know the correct action, direct or reverse, and fail position before starting on the Control Valve.

Control Valve 

Read More

Final Control Element: Control Valve

A control valve is simply a variable orifice that is used to regulate the flow of a process fluid according to the requirements of the process. 

 In a control valve, an actuator that is connected to the valve’s plug stem and moves the valve between the open and closed positions to regulate flow in the process. The valve body is mounted in the process fluid line and is used to control the flow of fluid in the process. The body of a control valve is generally defined as the part of the valve that comprises the main boundary, including the connecting ends. Valves are classified into two general types based on the movement of the valve’s closure part: linear and rotary.

Globe valve.

Types of Control Valves.

Though there are many kinds of valves, the most common types are globe, gate, diaphragm, butterfly, and ball valves.

The globe valve, which is of the linear movement type, is most common of these five types. In a globe valve, the plug is attached to a stem, which is moved linearly in a cavity with a somewhat globular shape to regulate flow (see Figure ).

A flat or wedge-shaped plate that is moved into or out of the flow path to control flow characterizes the gate valve. These valves are widely used for manual on/off service, but a few designs are used in throttling service.

-->
Diaphragm valves are linear-motion valves with flexible diaphragms that serve as flow closure members. Diaphragm valves are mainly used with difficult fluids such as corrosive liquids or slurries. The valve body can be lined with glass, plastic, or Teflon. The diaphragm is normally rubber, but in some cases it is Teflon, which, however, requires a high closure force.

The butterfly is by far the most common rotary-motion control valve. Butterfly valves range in size from one-half inch to over two hundred inches. In the very large pipe sizes, the butterfly valve is the only cost- effective
solution for the control valve application.

The ball valve is also a rotary-motion valve. The part that closes the flow is a sphere with an internal passageway. The ball valve is the most widely used control valve after the globe valve. Advances in seal design and sealing material enable the ball valve to offer tight shutoff. Because of this feature it is now widely used in on/off service for batch processes.

Read More

Distributed Control System or DCS: Brief History

Because of the reliability problems and high cost of the control process  computer systems of the 1960s, there were few new process computer projects in the early 1970s. The rare projects that were started in this period were based on medium-priced minicomputers that were designed to be small in size.  At the same time, two developments occurred in electronics that profoundly changed the application of digital computers to process control.

The first was the development of integrated circuits and microprocessors. The second was the release of the distributed control system (DCS) by Honeywell in 1969. This new design concept was based on the idea of

widely distributing the control to computer modules. Each of these modules controlled several instrument loops, generally one to four. They were connected by a single high-speed data communications link, called a data highway, that made possible communications between each of the computer modules and the central operator console. This design allowed the operator to monitor the operation of each local process.

In the mid-1970s, microprocessor-based modules replaced hardwired computer modules. The typical DCS had the configuration shown in Figure1. Today’s distributed control systems are much more powerful and

faster than the first systems because of improvements in microprocessors and other electronic circuits.

Fig. 1: Typical DCS configuration.

Distributed control systems today consist of one or more levels of control and information collection, as shown in Figure 2. The lowest level is process control and measurement on the plant floor. At this level, microprocessor- based controllers such as programmable controllers execute loop control, perform logic functions, collect and analyze process data, and communicate with other devices and to other levels in the system.

In Figure 2 , the process data collected at level 1 is transferred to level 2. At this level, process operators and engineers use operator consoles that have a keyboard, mouse, and video display to view and adjust the various processes being controlled and monitored by the system. Also, at level 3, process and control engineers implement advanced control functions and strategies, and members of the operations management team perform advanced data collection and analysis on process information. The various plant management systems—such as inventory management and control, billing and invoicing, and statistical quality control—exist at level 3. The highest level (level 4) is used in large industrial plants to provide corporate management with extensive process and operations information.

Fig. 2: Distributed Control System Level

Read More

Pressure Measurement: Gauge and Absolute Pressure

Absolute pressure is the pressure measured above total vacuum or zero absolute, where zero absolute represents a total lack of pressure.

Gauge pressure is the pressure measured above atmospheric or barometric pressure. It represents the positive difference between measured pressure and existing atmospheric pressure.

Most pressure gauges and other pressure-measuring devices indicate a zero reading when the measuring point is exposed to the atmosphere. This point is called zero psig. In fact, most pressure instruments actually measure a difference in pressure. However, some instruments are designed to produce a reading that is referenced to absolute zero and to indicate a reading near 14.7 psi at sea level when the pressure point is exposed to atmospheric pressure. This reading is generally termed psia. Figure 1 illustrates the relationship between absolute and gauge pressure.

The equation for converting from gauge pressure (Pg) in psig to absolute pressure (Pa) in psia is given by the following:

Pa = Pg + Patm (when Pg > Patm) (1-A)

Pa = Pg - Patm (when Pg < Patm) (1- B) where Patm is atmospheric pressure.

It should be noted that a change in atmospheric pressure will cause a change in gauge pressure. Therefore, a change in barometric pressure will cause a change in the reading of a gauge-pressure-measuring instrument.

This principle can be best illustrated by Examples 1-1 and 1-2.


EXAMPLE 1-1

Problem: If a pressure instrument has a reading of 30 psig, find the absolute pressure if the local barometric reading is 14.6 psi.

Solution: Since Pg > Patm, use Equation 1-A  to find the absolute pressure:
Pa = Pg + Patm
Pa = 30 psi + 14.6 psi
Pa = 44.6 psia


EXAMPLE 1-2

Problem: Find the absolute pressure if a vacuum gauge reads 11.5 psig and
the atmospheric pressure is 14.6 psia.

Solution: When dealing with pressure below atmospheric pressure, you must
use Equation 5-B:
Pa = Pg – Patm
Pa = 11.5 psi – 14.6 psi
Pa = – 3.1 psia

Read More

pH Measurement Applications.

Applications for pH measurement and control can be found in waster treatment facilities, pulp and paper plants, petroleum refineries, power generation plants, and across the chemical industry. In other words, continuous pH analyzers can be found in almost every industry that uses water in its processes. Figure 1 shows an example of pH control—a P&ID of the manufacturing of disodium phosphate using flow and pH control. The automatic control system shown in the figure produces a high-purity salt and prevents

 Sample P&ID:  Disodium Phospate Mfg. 

unnecessary waste of both reagents (i.e., soda ash and phosphoric acid).

Yokogawa PH450 EXAxt Analyzer for pH and ORP

Read More

What is Instrumentation?

When you say Instrumentation, for some  not involve in this field of industry doesn't really know what is instrumentation meant to be? 

According to Instrumentation and Systems Automation Society or commonly known as  ISA defined, Instrumentation is a collection of instruments and their application for the purpose of  observation,measurement and control.

An instrument  is a device that measures or manipulates variables such as flow, temperature, pressure, or level , include other devices which can be simple as valves and transmitters, and  as complex as analyzers. Instrument often comprise control system of varied processes, the control of these processes is one of the main branches of the applied instrumentation. 

• Process - Any operation or sequence of operations involving a change of energy, state,composition, dimension, or other properties that could be defined with respect to a datum.

• The sensor is also known as a detector or primary element. That a part of a loop or instrument that first senses the value of a process variable, and that assumes a corresponding, predetermined, and intelligible state or output. The sensor could be separate from or integral with one other purposeful element of a loop. 

• Controller -  A device having an output that varies to regulate a controlled variable in a specified manner. A controller may be a self-contained analog or digital instrument, or it may be the equivalent of such an instrument in a shared-control system.

• Final control element - the device that directly controls the value of the manipulated variable of a control loop. Often the final control element is a control valve.

Read More

Analytical Measurement: Turbidity Analyzer

A typical application of photodetectors in analytical measurement is as a turbidity analyzer. The cloudiness of a liquid, called turbidity, is caused by the presence of finely divided suspended material. Turbidimetric methods involve measuring the light transmitted through a medium.

Turbidity can be caused by a single substance or by a combination of several chemical components. For example, the amount of silica in liquid may be determined in approximate concentrations of 0.1 to 150 ppm (parts per million) of SiO_2. Sometimes composite material turbidities are expressed as being equivalent to silica.

In the typical application, a turbidity value is developed from a test sample under controlled conditions. In this application, a laser beam is split and passed through two mediums to matched photodetectors. One medium is a carefully selected standard sample of fixed turbidity. The other medium is an in-line process liquid. If the in-line process liquid attenuates the laser beam more than the standard or reference sample, the electronic circuit triggers an alarm or takes some appropriate control action to reduce turbidity.

Read More

A Simple Instrument Model.

The physical process to be measured is within the left of the determine and the measurand is represented by an observable physical variable X . Be aware that the observable variable X need not necessarily be the measurand but simply associated to the measurand in some known way. For instance, the mass of an object is commonly measured by the method of weighing, where the measurand is the mass but the bodily measurement variable is the downward force the mass exerts within the Earth’s gravitational field. There are a lot of potential bodily measurement variables. Figure 1. presents a generalized model of a simple instrument.

   The important thing practical aspect of the instrument model proven in Figure 1. is the sensor, which has the function of converting the physical variable input right into a signal variable output. Signal variables have the property that they are often manipulated in a transmission system, similar to an electrical or mechanical

circuit. Because of this property, the sign variable could be transmitted to an output or recording devices

that may be distant from the sensor. In electrical circuits, voltage is a common signal variable.

 In mechanical methods, displacement or drive are commonly used as signal variables. Other examples of

signal  variable are shown in Table 1. The signal output from the sensor might be displayed, recorded, or

used as an input signal to some secondary device or system. In a primary instrument, the signal  is transmitted

to a display  or recording device where  the measurement may be read by a human observer. The observe

output is the measurement M. There are a lot of varieties of display devices, ranging from simple scales and

dial gauges to sophisticated computer display systems. The signal will also be used directly by some larger

system of which the instrument is a part. For instance, the output signal of the sensor could also be used as

the input signal of a closed loop control system.

If the sign output from the sensor is small, it is generally essential to amplify the output shown in Figure 2. The amplified output can then be transmitted to the display machine or recorded, depending on the actual measurement application. In many instances it is vital for the instrument to supply a digital signal output in order that it could possibly interface with a computer-based mostly data  acquisition or communications system. If the sensor does not inherently present a digital output, then the analog output of the sensor is converted by an analog to digital converter (ADC) as shown in Figure 2. The digital signal is typically dispatched to a computer processor that may show, store, or transmit the info as output to some other system, which can use the measurement.

Read More

Flare gas mass flow measurement.

In a variety of oil production and refining environments, FCI’s GF90 Series mass flow meters monitor total gas flow to flare stacks as well as through feed lines to the main flare header. As an integral part of energy conservation and emission control systems, it meets strict accuracy and wide turndown requirements. The GF90 is highly reliable because of its durable construction and no moving parts design. Units are also used to assist in the control of steam addition and blower and louver adjustment for smokeless flares. A packing gland option permits hot-tap installation and retraction of the flow element without system shut down. The GF90 also features high flow range turn down ratios up to 1000:1 and micro-processor based user friendly electronics.

Read More