Make the switch

NGL fractionation process facilities separate ethane, propane, butane and other heavier hydrocarbons from the mixed NGL streams (Figure 1). Multiple trains and columns are used in this fractional distillation process at each of the facilities, with relief valves installed as critical safety protection devices that release when needed to prevent pressure from building within the fractionation process.

Figure 1. NGL fractionation processes.

A flow switch is typically installed after the pressure relief valve and alarms if there is flow. For the NGL fractionation process to work efficiently and safely, the operation requires early and reliable indication when a relief valve leaks or lifts during an overpressure situation.

Should the flow switch alarm due to gas being released through the relief valve, it indicates via a dry contact that there is an event in progress. The facility’s staff then responds by taking the appropriate corrective action to resolve the event. A company with multiple NGL fractionation process facilities, located in the Southeastern US, became concerned about a series of missed events and nuisance alarms caused by flow switches that were installed to monitor potential overpressure situations with its pressure relief valves. The company wanted a highly reliable flow instrument solution that would detect a real event and avoid the lost productivity of nuisance alarms.

The problem

Two different types of flow switch technologies had been tried unsuccessfully in the relief valve monitoring application by the company. Each type of switch failed to alarm when expected to do so and also caused nuisance false alarms. Failing to find the right solution was time consuming and expensive.

Figure 2. FLT93L flow switch.

Not all gas flow sensing technologies are able to detect the required low velocity of leaking or seeping gas required in this application. Differential pressure (dP) and vane type technologies, for example, had been tried by the company with limited success. The dP device relies on pressure to indicate as to whether or not there is flow, only ‘inferring’ that flow is taking place. Meanwhile, mechanical devices, such as vanes, can stick or freeze and are typically not sensitive enough at very low flow rates.

The solution

The company contacted the applications team at Fluid Components International (FCI) and requested a review of its problems. A representative visited the plant that was experiencing ongoing problems for a first-hand look at the fractionation process lines and discussions about the problems experienced during operations. FCI then recommended the installation of the FLT93L inline flow switch (Figure 2). This is a heavy duty thermal dispersion flow/level/temperature switch that was designed based on the company's experience in similar applications.

Figure 3. Thermal dispersion flow sensing.

This switch is designed with an all-welded thermal sensing element and includes an advanced electronic control circuit, which is field configurable to satisfy any combination of application requirements. The thermal sensor in this switch is suitable for both gases and hydrocarbon-based liquids.

A thermal dispersion technology flow switch offers several advantages, including a reliable performance that would both alarm when there was flow present at the fractionation facilities and avoid any nuisance false alarms.

The company's engineers decided to install the new switches at one plant for evaluation purposes prior to considering them for their other facilities. Each of their facilities would eventually need two or three flow switches, depending on the scale of operations at each plant.

Figure 4. Temperature-compensated sensor.

The pilot installation of the flow switches at the first facility was completed relatively quickly because it was a standard flow switch product available 'off-the-shelf'. Global agency approvals for Ex installations and a safety integrity level (SIL) 2 rating are also provided with the product.

The inline configuration flow switch was placed on 0.5 and 1 in. dia. pipes (DN15 and DN25). The switches were installed by local plant technicians on the backside of the pressure relief valves, and were integrated into the fractional process trains without any difficulties.

The flow switch's voltage output allows the user to ‘see’ into the process and accurately set the desired trip point. The delta range between the switch’s two resistance temperature detectors (RTDs) provides a span for setting the switch trip points. These flexible dual relays are settable by the plant technician for any combination of flow and/or temperature alarms.

With a standard flow accuracy of ±2% of the setpoint velocity over a ±50°F [±28°C] temperature range, the new switch met the needs of this application. Its repeatability of ±0.5% reading was also satisfactory for dependable, reliable operation.

Stainless steel enclosures were considered, yet a standard enclosure was chosen. It features a coated aluminium alloy and is rated for NEMA Type 4X (IP67) environments. The electronic control circuit can be integrally-mounted with the sensing element, or it can be remotely mounted up to 1000 ft away from the switch.

Thermal flow switches

Thermal dispersion flow switches are ideal for monitoring liquid or gas flow. Two platinum RTD temperature thermowells are used as the sensor. One RTD is heated while the other RTD measures the process temperature (Figure 3), whereby thermal flow switches are monitoring the customer’s direct variable of interest (flow) and can alarm, based on the cooling of the heated RTD.

A high voltage output value indicates a low flow or no flow condition, while a lower voltage value indicates that flow is increasing. Using this information, the user can ‘see’ into the process and set a numerical switch trip point with a high degree of confidence.

Figure 4 illustrates how temperature compensated thermal flow switches will not experience signal drift during temperature changes, whereas a non-temperature compensated flow switch experiences signal drift (as indicated by the red arrow) and could cause a false alarm.

Thermal flow switches are reliable because there are no moving parts to maintain or break free. For this reason, instrument life is also long (190 years mean time between failures [MTBF]) with a low lifecycle cost over what can be many decades of service. The company considered both the low maintenance and long life qualities of the thermal switch solution to be critically important. Integrated with this switch’s thermal flow sensor is a fail-safe, dual alarm single pole double throw (SPDT) control circuit for field flexibility and user friendliness. This unique FlexSwitch control circuit offers a variety of field-selectable features for many different applications.

The control circuit’s dual independent heavy duty 6A SPDT relays provide the multiple alarm combinations. They can be set for flow rate and temperature, high flow and low flow, point level and temperature, flow rate and low liquid level, three phase interface or fail-safe flow, level or temperature.

Conclusion

In the first two months of operation at the initial pilot installation facility, the flow switch correctly alarmed four times. The company verified the indicated relief valves were indeed leaking or seeping as designed to prevent overpressurisation. Since this original installation, the company has standardised on the FLT93 flow switch. It is now installed for this specific application and other similar applications at the company’s gas plants located throughout Texas, Louisiana, Oklahoma and Arkansas.

This article was originally published in Hydrocarbon Engineering. To receive your free copy, click here.