Field Instrument Calibration, Test and Troubleshooting - Term Paper Example

Field instrument calibration, test and troubleshooting Name: Institution: Tutor: Date: Introduction A variety of field instrumentations are widely used today in industrial and environmental applications. Various types of field instruments are applicable in a particular field site. This article will provide a brief theory of calibration of the common types of field instruments. For maximum safety and accuracy the user must ensure any instrument used at a field site is working properly and has been calibrated according the manufacturer's instructions. Along with a properly working instrument, a trained person who has an in-depth knowledge of the benefits and limitations of an instrument is the key to the successful application of field instrumentation. This article is going to focus on the calibration and trouble shooting of field instruments such as pressure transmitters, pressure switches, temperature sensors and flow elements. Calibration of pressure transmitters There are two adjustment made in the calibration of transmitter. It is made using the ZERO and SPAN of the potentiometers. These two adjustments are non-interactive: meaning that by changing one the other is not affected. The transmitter is wired in a complete loop. 1. Set the transmitter to a known zero reference point. The range of the transmitter may start at the atmospheric pressure of zero. Calculate the expected signal current in mA using a given formula, the formulas are usually in the manufactures manual. 2. Remove the cap of the transmitter and set the meter to DC mA, connect the positive terminal (Meter +) lead to the (DISP +) terminal. Connect the BLACK (Meter -) lead to the (DISP -) terminal. 4. LCD and the transmitter are made independently. 5. Adjust the transmitter ZERO potentiometer to zero. 6. Set the maximum range of the transmitter by exposing the transmitter to a process near the top of the range. 7. Calculate the reading expected using the formula provided. 8. Adjust the transmitter span potentiometer to the expected current output. CALIBRATION OF LCD DISPLAY NOTE: Be sure the transmitter is in calibration before attempting to adjust the LCD display. The LCD reading is based on mA output from the transmitter. Therefore, if the transmitter is out of calibration, this error will be reflected in the LCD reading. To perform calibration of the LCD display, you must first determine the Zero and Span. This information is on a sticker located on the back of the display. Adjustments are made via Zero And Span potentiometers, see Figure 5 for location. You may use either the RSP transmitter to perform calibration of the display, or a 4-20 ma signal simulator. CALIBRATION OF LCD USING 4-20 mA Simulator 1. Disconnect he LCD display from the transmitter by removing the RED and BLACK wires from their respective terminals. 2. Attach the POSITIVE lead of the simulator to the RED lead of the LCD display, and the NEGATIVE lead of the simulator to the BLACK lead of the LCD display. 3. The simulator should be set to POWERED output mode so that loop power is supplied. If your simulator is not capable of this function, wire to 9 VDC batteries in series with the 4-20 mA simulator and the LCD display. 4. Apply 4.00 mA to the LCD display. 5. Adjust the potentiometer to ZERO until it matches LCD display. 6. Apply 20.00 mA to the LCD display. 7. Adjust the SPAN potentiometer until the he LCD matches the range indicated on the sticker in the spot labeled SPAN CALIBRATION. 8. The LCD display is now properly calibrated. Troubleshooting Voltage Check Test for the correct loop power using a digital multimeter. This is done on the DC volts scale with the sensor connected to the receiver. The red led of the meter is placed on the positive loop terminal, while negative terminal of the meter is placed on the negative terminal of the loop. It should be between 9-32 VDC for a standard transmitter. Current Check Make sure that the sensor is connected to the receiver. Expose the wiring terminals in the cap on the transmitter. Connect the positive lead of the multimeter to the positive terminal and the negative lead to the positive terminal. You will be reading the mA current loop. A high current flow, approaching 30 mA, indicates a problem with the transmitter (internal source). If no current flow is observed this indicates either an open loop or a problem with the transmitter. To check a transmitter at a know pressure; refer to the Calibration section for information on how to properly calculate the mA output at the known value. Miscellaneous Troubleshooting Symptoms Action No display on the receiver Check the receiver for power out Check the receiver for power out Display on receiver improperly Perform calibration check on signal receiver When testing loop no current flow Check the broken connections Low reading for pressure Check for the incorrect polarity Check test equipment Calibrating pressure limit switches The first step in the process is to set up the DPC. (Note: A number of the terms in this article apply to both temperature and pressure limit switch calibration and maintenance.) The DPC test setup screens prompt the user for the following information: • Setpoint: Main point at which the switch is supposed to take action. • Setpoint type: Can be “high” or “low.” This is the basic call to action. “Low” means that the action should happen when the process variable (PV) is below the setpoint. “High” means that the action should happen if the PV is above the setpoint. • Set state: State of the switch (set or reset) at the time the action takes place. • Tolerance: The allowable deviation from the set point. • Deadband min: Minimum value or size of the dead band. • Deadband max: Maximum value or size of the deadband. (Note: the deadband of a pressure switch is the measured difference in the applied pressure when the switch is changed from set to reset) Calibrating pressure switches with a DPC • Trip function: This can be set for continuity, V ac or V dc, and refers to what is being measured as the setpoint is exercised by the simulated process variable. For example, suppose you want to control the pressure in a vessel set at 12 psi. You don’t want the relief valve to be opening and closing constantly, you want it to open at 12 psi and close again at approximately 10 psi, (12 psi - 10 psi = an approximate deadband of 2 psi Temperature sensors Temperature sensors are not usually calibrated in the field because they are already calibrated by the manufacturers. The accuracy of the temperature sensor can be verified in the field. If a field instrument contains several sensors, each sensor must be verified for accuracy. For example to verify a temperature sensor for measuring a water bath, a container is filled with water and the temperature of the water below the water body’s temperature to be measured. Ice or warm water is used to vary the temperature accordingly. A thermometer and the temperature sensor are placed in the water bath and wait for both temperature readings to stabilize. The measurements of both readings are compared. The temperature sensor readings must be the same with that of the reference thermometer and within the accuracy of the sensor. The water temperature is then adjusted to a temperature higher than that of the water body and the same procedure is repeated again. Flow elements and Transmitters. Bernoulli established a relationship between kinetic and static energy in a flowing stream. In his, he found out that as a fluid passes through a restriction, it accelerates, and the energy for this acceleration is from the fluid's static pressure. Consequently, the pressure drops at the point of constriction (Figure 2-1). Pressure drop is recovered as the flow returns to the unrestricted pipe. The pressure differential (h) of the flow element is measured, and the speed (V), the volumetric flow (Q) and the mass flow (W) can all be determined using the following formulas. V = k (h/D)0.5 or Q = kA(h/D)0.5 or W = kA(hD)0.5 Where K is the discharge coefficient A is the cross-sectional area of the pipe D is the density of flowing fluid Discharge coefficient is affected by the Reynolds number and the ratio between bore diameter and the diameter of the pipe. The discharge coefficients of primary elements are accurately determined in the laboratory. The average values of coefficients can be determined by running some reasonable calibration runs. Published discharge coefficients can also be used to accurately determine flow measurements without calibration. At low Reynolds numbers, the flow is laminar and the velocity profile is parabolic whereas at high Reynolds numbers the flow becomes fully turbulent, and the resulting mixing action produces a uniform axial velocity across the pipe. As shown in Figure 1-5, the transition between laminar and turbulent flows can cover a wide range of Reynolds numbers; the relationship with the discharge coefficient is a function of the particular primary element. Tuning a control valve is adjusting its control parameters up to optimum values in order to achieve a desired control response. The controller is switched to manual when there are no expected plant upsets and that the process is practically stable. Then D, which stands for derivatives or rate on the controller, is set to minimum and I, which stands for integral or reset on the controller is set to maximum. A set point that is equal to the measurement is then selected and the proportional band adjusted to 100% or at 1.0 gain to start. The output is then changed to a small amount and the controller transferred to automatic while noting the starting valve position. Incase oscillations don’t develop then step 2 is repeated while reducing the proportional band in order to halve the value that was tried before until oscillations start. Incase oscillations which have increasing amplitude form on the first try then one should go back to manual and the valve set at the original position as in step 2.Try again after doubling the proportional band until uniform oscillations occur and measure the period. For a P+1 controller, I is set to the period x 0.82 and the proportional band doubled. The period will increase by around 43%.The proportional band is readjusted if more or less damping is required. If I is set to the period x O.S then D is set to the period x 0.12 and the proportional band is doubled, the period decreases by about 15%.The proportional band is readjusted if more or less damping is required. It should be noted that large I and small D values are the safe values. Default tuning parameters are always available for the start-up of all plants and they can be entered into each controller. They are usually start-up values only and additional tuning is still required for each controller. For start-up conditions, there are usually no values shown for Derivative action since Derivative values are usually added during the final tuning of the appropriate controllers. CONCLUSION In conclusion, the above processes show step by step instruments calibration, which done according to the set standards by comparing the calibrated instrument to the standard one. The process of instrument calibration can be done manually but can also be done through an automatic calibration process. For example analogue instrument is calibrated manually unlike digital instrument. Calibration is done regularly to ensure that the instrument has minimal errors as much as possible. Read More