Side Mount vs. Top Mounted Level Instrument

 

Side Mount vs. Top Mounted Level Instrument

What should be considered when deciding whether a level device (transmitter or switch) should be top or side mounted with an external cage?

Top mounting means that the sensor or measuring element of the level gauge is inserted into the container or tank from above. Side mounting with an external cage means that the sensor or measuring element is integrally installed in an external cage prefabricated by the manufacturer.

The price of the external cage leveler is higher than the top mount type. This is due to the cost of the cage, in addition for a special material such as Duplex Stainless Steel. However, an external cage level gauge may have an isolation valve installed between the tank and the cage. This arrangement allows maintenance of the device without the need to shut down the process. Therefore, an external cage level measurement tool is preferred in most applications.

In some cases, a top mount tool is used, such as:

A buried ship because it has no side access.

A semi-submersible tank such as a caisson.

A liquid that can change properties/form due to temperature change and the use of steam or electrical auxiliary heating is not possible.

Why must the cable shield not be grounded on the instrument field device?

Grounding of the device cable shield

Why must the cable shield not be grounded on the instrument field device?

Cable shielding protects the signal transmitted by the conductor from external interference. The cable screen blocks external interference and noise and then directs it to the ground.

The screen must be grounded to only one source. Grounding is usually done in the equipment panel or cabinet, while the field equipment point is left ungrounded and isolated. This eliminates the ground loop current that can occur if the shield is grounded at both ends. In this case, the ground loop current becomes noise for the signal wire itself.

The following wiring diagram shows the ungrounded instrument cable shield on the operating device.

INSTRUMENT RANGE SELECTION

 Choice of instrument range

The range of the instrument is chosen based on the operating range of the process to be measured. In common practice, it is recommended to select a full-scale instrument that can provide an operating reading between 30% and 70%, especially for a pressure gauge. The following is an example of selecting a pressure gauge range

Given process data:

Minimum operating pressure: 5 kg

Maximum operating pressure: 9.5 kg

Maximum design pressure: 25 kg

Available standard pressure gauge range: 0-10 kg, 0-16 kg, 0-25 kg

[X] A selection of 0-10 barg will provide a 95% reading during maximum operating conditions. The device has only a small reserve above the maximum operating pressure.

[√] Selecting a range of 0-16 barg will provide the best reading during operating conditions. However, the maximum gauge pressure is normally 1.3x the maximum scale, which is only 16×1.3 = 20.8 barg. In such a case, the gauge must be equipped with over-range protection to withstand the design pressure.

[X] The 0-25 kg selection can withstand up to 32.5 kg (25 x 1.3) without using overrange protection, but under normal operating conditions this would give a poor reading indication.

Shutdown logic diagram

 Schematic of shutdown logic

A shutdown logic diagram (also known as an ESD logic diagram) shows the shutdown level hierarchy within a plant or platform. The shutdown logic diagram starts by telling the big picture of the shutdown level, usually the level starts from 0 and continues with a higher number (1, 2, …). A lower number represents a more critical shutdown. The following sheets continue with a hierarchy of details for each process unit.

A trip logic diagram describes the 'cause' that can trigger a trip and the effect that can occur after the signal is activated. This diagram also shows the action such as time delay, start inhibit, reset that will be performed for each action.

The location of each instrument acting as a trigger/cause must be clearly specified.

Reference document: SAFE Chart, philosophy

The SAFE table lists the required safety devices for each process device. The SAFE table must ensure that all safety requirements have been met and what executive actions the safety device must perform.

The philosophy provides the narrative and the engineer translates it into cause and effect interaction so that the plant works as the intended philosophy.

Purpose of the Shutdown Logic Diagram

The shutdown logic diagram can be used as a reference when preparing a cause and effect diagram.

Sensing Elements for pressure measurement

 Elements for pressure measurement

Bourdon tubes

A Bourdon tube is a metal coil made of a metal tube that is closed at one end and attached to a pressure source at the other. It has the required elasticity, due to which the pipe tends to deform in order to straighten itself under pressure. Applied pressure will result in movement at the free end causing the mechanical linkage to move the pointer. Bourdon tubes come in several forms, i.e. C tube, spiral or helix.

Bellows

The pressure element of the bellows is made of seamless tubes that are rolled. It looks like thin-walled corrugated cylindrical containers. The corrugator is flexible along its axis so that it changes its length in proportion to the applied pressure.

Diaphragm

A diaphragm sensor is a thin flexible metal disc. Pressure applied to one side of the disc causes a deflection that activates the pointer. To measure very low pressures, the diaphragm must be very thin. In this case, it is difficult to manufacture and its stability is poor. Capsules can be used for this application. It is made of two diaphragms welded together, so it doubles the deflection of the diaphragm, allowing smaller pressures to be measured without reducing its performance.

Use of temperature transmitters

Use of temperature transmitters  

In order to create a temperature measurement system, an electronic temperature measuring device must be connected to the control system. The RTD and thermocouple can be connected directly to the I/O card of the control system or through a temperature transducer for signal conversion. In most applications, a temperature transmitter is used to convert the RTD / thermocouple signal and transmit the 4-20 mA signal, rather than connecting the RTD or thermocouple directly to the I/O card

Choosing to use a temperature sensor over directly connecting an RTD or thermocouple to the control system provides many advantages:

Signals generated by a resistance temperature detector or thermocouple are categorized as a low-level signal and are therefore more susceptible to noise compared to a conventional 4-20mA signal generated by a transmitter.

Reduce the device cable type. It also means less type for a replacement cable.

The junction box for temperature sensor cables can be combined with cables from other 4-20 mA sensors.

There is no need to provide a special I/O card for the thermocouple/RTD. Just connect the transmitter to the analog input I/O card.

It provides a maintenance facility such as a diagnostic function in a smart transmitter.

Temperature element Instrument

 Temperature element Instrument

There are several types of temperature elements widely used to measure the temperature of a process substance. These types are divided into mechanical temperature elements and electronic temperature elements. The mechanical thermal element uses the principle of thermal expansion, that is, the element expands when the temperature increases and vice versa. The electronic temperature element uses the principle of changing electrical properties following a change in temperature.

Mechanical temperature element

Solid element

A bimetallic is a thermal element created from two types of metals that have different thermal expansion properties. The two metals are joined together to form one strip. Metal strips bend when the temperature changes. An indicator is attached to the free end of the bimetal to indicate the result of the temperature measurement.

Gas element

A gas-filled system is a temperature element based on the principle of the ideal gas law. It consists of a bulb/stem, a capillary and a Bourdon tube. The sensing element is a solid bulb or spindle containing gas. If the temperature rises, the volume remains constant because the flask is rigid, while the pressure of the gas increases proportionally. The change in gas pressure is measured by a pressure element such as a Bourdon tube.

The flask and bourdon tube are connected by a capillary tube allowing the placement of a temperature indicator that is not directly connected to the pipe/vessel being measured. These advantages allow the thermometer indicator to be mounted in a convenient location, while the sampling point of the measured process fluid is not permanently inaccessible.

However, the capillary can lose heat, so compensation must be added to eliminate the error. www.instrumentationportal.com

Liquid element

The liquid temperature element uses mercury in the bulb/stem. Mercury is no longer preferred in most process applications, although it provides fast response and good accuracy. Mercury is mostly used in a glass stem thermometer for non-process industries such as measuring body temperature.

Electronic device for measuring temperature

The two most commonly used electronic temperature elements are the resistance temperature detector (RTD) and the thermocouple. To indicate the temperature measurement, these elements need to be connected to the control system by cable directly or by means of a temperature sensor.

Resistance temperature detector

The Resistance Temperature Detector (RTD) works on the principle that the resistance of a metal changes with its temperature. RTD is the most commonly used type in process measurement due to its good stability, accuracy, repeatability.

Thermocouple

A thermocouple consists of two different metals, the transition of which generates a voltage proportional to the transition temperature. The thermocouple is selected for operation that requires a wide range (very high - low temperature). Other advantages of a thermocouple over an RTD are that it has a more durable construction and also provides a faster response.