Case Study of Common Problems with Measuring Instruments

Measuring instruments is widely used in varies industries as fundamental data acquisition tool in their production process. Common problem from measuring instrument can be data reading inaccurate (Either higher or lower than standard), no data reading, unstable data reading and so on. The cause of these problems is generally either bad quality, faulty parts or outside effects (such as working environment). However, it’s very hard to identify exactly what is the main reason for faulty reading. From this article we will looking at different case studies on the reason of faulty reading of different type of measuring instruments.

From below situation we will be able to identify whether the cause of faulty reading is from bad quality or faulty parts:

• We should be able to identify there might be a faulty part from our instruments When recorded measuring data are exactly the same for a certain period.
• The most common control system in measuring instrument is Distributed Control System (DCS). This system is very sensitive and able to show any small changes from recorded data. To identify if there is an issue with the system, operator only need to change one random setting from instrument to see if there are any changes.
• Compare the unnormal instruments with the same one onsite can also tell if there is a faulty situation in the instrument.

Case Study:

Flow Meter:

1. Electromagnetic flowmeter shows zero in display (reading always at 0);

The application of electromagnetic flowmeter in chemical production line is to monitor the electromagnetic wave generated by the chemical production process. In order to ensure that the electromagnetic wave generated by the operation of chemical equipment is in a controllable range.

Problem Identification:

1. Make sure there are liquid flowing through the pipe;
2. Identify flow direction is matching with flow meter;

• Identify is the measuring pipe is fully filled with testing medium. Also check if there are bubbles in the line as well;

1. Check if the transmitter is disconnected with electrode;
2. Check if all specification requirements are set;
3. Check if there are any signs of environmental or physical damage;

2. Differential Flow transmitter display value has big difference with real value;

Problem Identification:

1. Connectingpipe might be blocked；
2. When measuring corrosive gas, rust residue might be stored in the connecting pipe;
3. If measuring medium is a mixture of liquid and gas, there is a chance that the liquid is frozen in winter;

1. Threevalves manifold might be blocked;
2. There might be a leakage (broken O-Ring) at the three valves manifold;
3. The leakage may also be caused by ventilation between high-pressure and low-pressure chamber;

Pressure Transmitter

1. Flush diaphragm pressure gauge or pressure transmitter with flange connection has inaccurate display value;

• The most possible reason for inaccurate reading occurred is from the flange connection. The O-ring or gasket might be pressing on the diaphragm to cause inaccurate reading displayed.

2. In the pressure measuring pipe of a cracking gasoline pressure detection system in a petrochemical enterprise, the pressure indication returns to zero, the control valve is closed, the cracking tower does not discharge material, and the incident is caused by the high liquid level in the tower.

• Because the pressure of the system fluctuates greatly during normal operation, the method of opening the pressure valve and controlling the resistance with the needle valve can reduce the fluctuation of the instrument indication. Because the diameter of the pressure valve is relatively large, it is difficult to control, once the pressure valve is closed small to the pressure indication fluctuation within the allowable range, in fact, the valve is basically in the fully closed position, and usually did not notice this problem, when the insulation is closed, that is, the indication is zero, the control valve is fully closed phenomenon. The reason is that after the thermal insulation steam is closed, the pressure guide tube is cooled, and the original vaporized medium in the pressure guide tube is condensed into a liquid volume, and the pressure drops sharply (almost to zero), such as the pressure valve is not dead, the medium condenses into a liquid volume, and the medium in the tower is supplemented to the pipe, so that the pressure of the instrument is consistent with the pressure in the tower. The valve is closed into a closed container, because the medium is in a state of vaporization when heat preservation, the pressure is high or unchanged when the temperature becomes low, the medium liquefaction pressure becomes smaller, indicating a return to zero. The signal of the instrument is zero, and the control valve is completely closed through the regulator, which causes the liquid level of the tower to rise rapidly and cause the incident;

Temperature Transmitter

Abnormal temperature indicator usually means that the indicator is too high, too low or changes slowly or even does not change. When analysing the fault of temperature control instrument system, we should pay attention to two points: the system instrument is mostly measured by electric instrument; The measurement of the instrument indicating and controlling the system often lags behind.

The indicating value of the temperature instrument system suddenly changes to the maximum or minimum, generally for the instrument system fault, because the temperature instrument system measurement lag is large, there will be no sudden change, and the reason for the fault at this time is mostly thermocouple, thermal resistance, compensation wire break or transmitter amplifier failure.

The temperature control instrument system indicates rapid oscillation, which is mostly caused by improper PID adjustment of the control parameter.

The temperature control instrumentation system indicates large, slow fluctuations, most likely caused by changes in process operation. If the process operation does not change at that time, it is likely that the instrument control system itself is faulty.

1. The common faults of industrial thermal resistance are open circuit and short circuit. Generally, the break is more common because the thermal resistance wire is smaller;

1. Open circuit and short circuit is easy to judge If the multi-meter indicates infinity, the resistor body can be determined to be disconnected. The short circuit of the resistance body is generally easier to deal with, as long as it does not affect the length and thickness of the resistance wire, find the short circuit to blow dry and strengthen the insulation. It is necessary to change the length of the resistance wire and affect the resistance value, so it is better to replace the new resistance body.

Inconclusion, to identify the reason of inaccurate reading from measuring instrument is important to help operator or technician to apply possible solutions to solve the problem. The best way to figure out is to consulate with instrument manufacturer.