Accuracy vs Inaccuracy

Instrument engineer usually finds term ‘accuracy’ in some catalog and ‘inaccuracy’ in some others. To know whether there is a difference between both of them, try to understand the meaning of each.

The accuracy of an instrument is a measure of how close the output reading of the instrument to the correct value. Inaccuracy does mean similar i.e. the degree to which a measurement might be error from the correct value.

Let’s take a measurement device with 0-100 psig range and 2% of span accuracy. It does not mean the error will be 98 psig, instead the error will not be more than 2 psig. Although in this condition the term inaccuracy could be more suitable, yet most industry practice express the error reading from correct value as accuracy. The term accuracy and inaccuracy is often used interchangeably. READ MORE >>

Flow Orifice Turn Down Ratio

Orifice, combined with differential pressure transmitter, is the mostly used flow measurement device in oil & gas plant because of low cost and its ease of installation and maintenance. However, orifice only allows 3:1 rangeability to maintain accuracy. This means, if one wants to measure maximum flow of 10 MMscfd, then the minimum flow measurement that could be measured is only 3.3 MMscfd.

According to Bernoulli principle, the relationship between flow and pressure drop of fluid passing through an orifice is given by the following formulae:

Q = Volumetric flowrate
dP = Measured pressure drop ρ = Density
Ao = Cross-sectional orifice area
Cf = Constant

Hence, flow value could be obtained by measuring pressure drop and the relationship between them is square root.
Let’s say the orifice is intended for flow measurement of 0-10 MMscfd which is represented by 0-100 inH2O pressure drop (as show on below curve)


At maximum flow of 10 MMscfd, a change of 1% of full flow (0.1 MMscfd) to 9.9 MMscfd will be represented by a change of 2% of dP (from 100 inH2O to 98.01 in H2O). The following table summarized the value of flow, its corresponding pressure drop, and its dP change in percentage of full scale for 1% change of full flow at the specified flow. The less flow to be measured, the more sensitivity suffers.


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Actuated Valve Safety Factor

During designing an actuated valve, instrument engineer shall concern to valve safety factor which is a ratio between “torque produced by an actuator” to “torque required by a valve to actuate when changing position” (close to open or vice versa). Valve and actuator commonly obtained from different manufacturer, therefore it is a responsibility of instrument engineer to ensure that combination of the selected valve and actuator will operate properly and meet the safety factor specified by project.

To obtain the information simply doing the following step.

After deciding valve size, valve rating, valve material, etc., go to valve catalog and select one valve that meet the specification. Then, obtain valve torque data. Valve torque is affected by maximum differential pressure across valve (Max dP). This value is determined by process department and shall be informed to vendor. Max dP usually occurs when valve is fully close so that one side at maximum pressure while the opposite site is at no process fluid condition.

Afterwards select an actuator that can produce torque higher than the required valve torque. Note that torque produced by an actuator is affected by power source, e.g. instrument air pressure in pneumatic actuator.

Usually vendor will provided safety factor table along with quotation, following with engineering review. Below is one example of safety factor table:

(Click To Enlarge)

What is the recommended safety factor value? Many projects require 1.5 or 2. Note that safety factor which is too high could lead to miss-operation of an actuated valve.

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Loop Mode (2): Energized to Safe

Energized to safe mode works as contrary to that of de-energized to safe. In energized to safe mode, no electrical current flows through the instrument loop during plant normal condition. Loop will be energized during plant upset.

For illustration, see below picture of a simple unit consisting push button (PB) and solenoid valve controlling a deluge valve (XV). During plant normal condition (no fire) the push button contact is at open position so that no current flows within the loop. Similarly, the solenoid valve of deluge valve is not being energized by electrical power. This condition drives the deluge valve to close position, prevents the fire water from ring main passing through the deluge valve.


If there is a fire in deluge coverage area, someone would activate the push button. Once activated, the switch within push button changes state to close position and enables electrical current flows within the loop. This current will signal the control system and execute the predetermined action i.e. to open deluge valve by energizing the solenoid valve loop. Therefore, once the solenoid valve energized, it will make the deluge valve to open, allowing water flows through to extinguish fire.

This mode is also commonly applied for ESD push button for shutting down a plant.

The purpose of having energized to safe system in such system is to avoid misoperation of final element (activation of deluge valve on above example) due to instrument failure or unintended instrument cable disconnection.

Imagine if this system applies de-energized to safe mode instead. Even only one failure on solenoid valve cable or system side fault e.g. I/O card, will cause the deluge to open automatically. Not only will this cause the equipment including surrounding instruments to wet, but this will also be dangerous as the equipment within the area is still not electrically isolated.
Worse would happen in ESD push button, as it will cause a plant shutdown that results in production loss.

Further discussion of line monitoring in energized to safe mode will be in next post.

Additional Fact: Deluge valve is also activated by confirmed fire signal generated by flame detector, smoke detector or heat detector.

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Loop Mode (1): De-Energized to Safe

Electric based Instrument loop on plant operate in two modes: either energized to safe or de-energized to safe. De-energized to safe mode, also known as fail safe system means that during plant normal condition, there will be electrical current flows through the instrument loop. When trip/shutdown is required, the loop will be de-energized.

See illustration on below picture of a simple unit consisting Pressure Switch High High and SDV. During plant normal condition (pressure is lower than HiHi setting) the switch contact is closed and enables current flow within the loop. Likewise, the solenoid of SDV is energized by electrical power to allow air supply stroking the SDV and then forcing it in open position.

As long as the control system receiving current signal of pressure switch loop, plant is considered as normal. If the pressure increases and reaches HiHi setting, pressure switch will be open and there will be no current flows, hence alarming plant control system.The predetermined executive action shall then be taken to put system in safe condition i.e. de-energize the SDV’s solenoid to make the main valve close.

The reason de-energized to safe mode to be implemented in such system is to make it in safer state (in above case by closing the main valve) when either one of the following conditions occurs:
- Unintended instrument cable disconnection (either pressure switch or solenoid valve)
- Electrical black out

Energized to safe system will be discussed in other post (energized to safe system).

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Temperature Measurement: RTD or Thermocouple?

Two most common temperature instruments used in process industry are RTD and thermocouple. To determine which one to use, the following should be considered:

RTD is preferred if one of the following aspects becomes a concern in measurement: Accuracy, Stability, Sensitivity and Linearity.
Thermocouple is preferred in application for high temperature measurement (more than 400 degC) or when exposed to shock or vibration.

This is the reason why projects usually specify RTD for most process temperature measurement, while thermocouple is applied in heater, flare or for vibration monitoring of compressor or pump.

Cost? It depends on the installation. Some panel or I/O card does not accept RTD directly so it requires a transmitter. On the other hand, extension wire would be an additional cost of thermocouple.

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Instrument Power Cable Sizing

Sizing of instrument cable should be performed to ensure that instrument device such as solenoid valve and its cabling works properly. There are two factors that should be checked i.e. voltage drop and current carrying capacity.


Voltage drop is calculated as follows:

Vdrop = [Rio + (Rw x 2L) + Rins] x In

where:

Rio = Input resistance of I/O Card (Ohm)
Rw = Resistance of wire at specified temperature per lenght (Ohm/m)
Rins = Resistance of instrument device (Ohm)
L = Length of cable / distance from instrument to cabinet in control room (m)
In = Load current (A), obtained from power consumption of instrument device divided by power supply voltage.
Rins, Rio and Rw can be obtained from manufacturer’s catalog for instrument, I/O card and cable respectively.

The cable suits the application if calculated voltage drop is less than maximum permissible voltage drop by the system.

Current Carrying Capacity of a certain cable is defined as the maximum current that can flow through a cable without melting the conductor or insulation. Current Carrying Capacity varies depending on several factors such as conductor size, ambient temperature, and cable installation. These factors are referred as derating factors and IEC has established the values for each.The cable suits the application if nominal current (In) is less than current carrying capacity.
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Solenoid Valve with Manual Reset

When solenoid valve is losing power (de-energized) due to shutdown or other reason, it will actuate to its normal position. On regular solenoid valve, it can be put back to operating condition by energizing the solenoid. While on solenoid valve with manual reset, it will not be so. After being energized, it should be manually resetted prior back into normal operation. So it works like a local permissive.

The necessity of having a manual reset in solenoid valve should be determined during design phase, this shall be identified by process engineer or by operator through HAZOP. The philosophy is based on whether the operator is required to be present on site or not. He/she should check the surrounding condition and make sure everything on site is normal (e.g. no hydrocarbon spill) before resetting the valve. READ MORE >>

Pneumatic System Design to Enable Solenoid Valve Online Testing

There are two fails which are widely known in safety system. The first one is called a nuisance fail which does not put the plant system in danger. In fact some nuisance fails will result in spurious trip or unnecessary plant shutdown therefore they cost high as the production stops. The second one is fail danger which is undetected and does not cause a process shutdown. However, if there is an emergency demand, the safety system would be unable to respond properly therefore putting the plant system in danger.

Fail danger frequently occurs on the final elements i.e. actuated valve. One of several causes that make the actuated valve does not work properly upon demand is a failure on its solenoid valve (solenoid valve), either the solenoid coil or the valve. Dirt causes the solenoid valve to stick thus solenoid valve can not change state and unable to respond to emergency shutdown signal.

As mentioned earlier this situation is undetected, however the problem can be resolved by periodic maintenance which does not require shutting down the plant or actuated the main valve.

The following are design and accessories required to enable online testing of solenoid valve.

*Five-way valve shall be key protected to avoid unauthorized testing

During normal plant operation, the solenoid valve is energized, letting the pilot air flow through the solenoid valve, then it passes through the five-way valve and finally pilots the pneumatic pilot valve resulting the air supply is able to reach the actuator. In the meantime, pressure switch is also pressurized by pilot air.

When it is required to test the solenoid valve, five-way valve must be operated by key and hold until test is completed. In this condition, the solenoid valve is bypassed meanwhile the pressure switch is losing its pressure. When the pressure switch reaches its low set point, it signals the control room that the testing is in progress. Now the solenoid valve can be tested without affecting the main valve since the pilot air remains able to reach the pneumatic pilot valve and keeps the main valve in operating condition. By reading pressure gauge, check that solenoid valve is able to respond to emergency shutdown signal and changes its state).
Releasing the key will put the system into normal plant operation

*The purpose of installing a pressure swicth low is to avoid missoperation of
key operated override valve during plant normal operation.

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Completing The Actuated Valve

Actuated valve, also referred as shutdown valve, blowdown valve, on-off valve, mainly comprises of valve body and actuator. Here are several accessories that make actuated valve complete.

1. Solenoid Valve
Comprises of a solenoid and a valve which form is usually a three-way valve. Solenoid converts electrical energy to mechanical energy resulting three-way valve changes its state.

Here is one example how the solenoid valve working in actuated valve that operates in fail safe mode* as shown on above schematic:

Refer to the lower box in schematic when there is no electrical current received by the solenoid (in other word “de-energized”), the spring makes the valve in a state of the inlet port (1a) blocked while the outlet port (1b) connects to the vent port (1c), releasing air inside the actuator to the atmosphere.

If the solenoid is energized by electrical current, it will change the valve state. Refer to the upper box in the schematic, the inlet port (1a) connects to the outlet port (1b) allowing the air supply flowing into the actuator and stroking it, while the vent port (1c) is blocked.

*fail safe mode means that the actuated valve shall bring the system to safe condition when instrument air or electrical starts to fail. Shutdown valve (SDV)shall be closed, or referred as FAIL CLOSE, whereas Blowdown valve shall be opened or referred as FAIL OPEN. More description about fail safe mode, read this post.

In some application where there is no electrical power, pneumatic piloted valve is used in lieu of solenoid valve. It has the same principle as solenoid valve, but the power source is pneumatic instead of electrical.
Commonly used manufacturers: ASCO, VERSA, and BIFOLD.

2. Limit Switch / Position Transmitter
Position transmitter is used to remotely monitor valve opening from the control room. This instrument is attached on the actuator and transmitting analog electrical signals to indicate the exact position of valve opening. Meanwhile limit switch can only provides remote indication of valve status (open or close position) through electrical discreet signal.
Commonly used manufacturers: WESTLOCK

3. Air Filter Regulator
This device filters the air supply and regulates its pressure, resulting clean air supply with constant pressure fed into actuator.
Commonly used manufacturers: FISHER AFSR

4. Quick Exhaust Valve
Quick exhaust valve can increase the reaction of actuator spring when valve is required to fail safe. This is achieved by releasing the exerting fluid inside the actuator into the open line (port 4c), rather than back through the supply line (port 4a) which is more restrictive as it has to pass flow regulator valve and solenoid valve.
Commonly used manufacturers: VERSA

5. Flow Regulator Valve
To achieve the required closing and/or opening time of actuated valve
i. Uni directional flow regulator
This valve regulates flowrate of air supply in one direction only, while it doesn’t regulate the flowrate of air supply from other directions hence full flow will be passed through. It is used to set the opening time of SDV or closing time of BDV. During start-up the actuator is stroked with rate of movement depends on air supply flowrate flowing into the actuator. FRV will restrict the flow from (5.ia) to (5.ib) resulting the flow rate of air supply into the actuator decreases so that the actuated valve will move slower. If the actuated valve is required to fail safe, the air supply shall be vented from the solenoid vent port (1c). The uni-direction FRV will keep full flow from (5.ib) to (5.ia).
ii. Bidirectional flow regulator
Unlike uni-directional FRV, bidirectional FRV applies restriction to both directions. Installing this FRV will set the time required by the actuated valve to change its state to safe position.

6. Check Valve
It allows flow from one direction only i.e from (6a) to (6b). Commonly check valve is used to protect the upstream equipment from back flow.

7. Bug Screen/Silencer/Dust Excluder
This prevents bug or dust entering the pneumatic system from ports open to the atmosphere.

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