Differential pressure

Accurately determining the level of water/hydrocarbon interfaces is critical to the operation of equipment such as oil/water separators. Trevor Dunger argues that differential pressure measurement is the best of the available alternative methods.

Many processes use water as a means of transporting product from one point to another. For example, in oil production, water or steam is often used to lift oil out of a well.

In chemical production, water is sometimes a by-product or a tool used to clean a vessel. In these situations, the water and hydrocarbons will mix together. At some point, it will be necessary to remove the hydrocarbon from the water.

If allowed to settle undisturbed in a tank, the mixture will separate into its two components, with the heavier, denser material sinking to the bottom and the lighter, less dense material rising to the top.

One example of this is a separation tank. A control valve regulates the ingress of a liquid mixture of water and hydrocarbon into the vessel. Eventually, the lighter material in the mixture finds it way up to the separation stack, where a water/hydrocarbon liquid interface forms.

Critical position

The position of this interface is critical — too little or too much either way will end up with either water being drawn out with the hydrocarbon, or hydrocarbon remaining in the tank. In either situation, the end result is reduced product quality and process efficiency, adding to the product cost.

When the mixture gets to the critical interface point, a pump will pull out the hydrocarbon from the stack while a continuous amount of new mixture is pumped into the tank. The hydrocarbon is then sent on for processing, free of water.

For this process to operate at optimum efficiency, it is vital that the interface level is measured and controlled properly. A range of different technologies exists for interface level measurement applications.

Many of these can encounter problems when either the interface level becomes too small or the process involves sticky solids. Substances that can coat or leave residue can also present a problem when using these devices.

Here, we look at the advantages and disadvantages associated with the three main methods commonly employed for interface level measurement — displacers, capacitance probes and differential pressure transmitters.

Displacer type transmitters rely on the principle of buoyancy and consist of a large chamber flanged to the separation stack. A float or element of a known specific gravity will float at the point of interface. A series of moving-part linkages attached to the float indicate the float’s position to a transmitter, informing it of where the interface is.

Although relatively straightforward, this technique has a number of key disadvantages. Firstly, petrochemical and chemical applications are often characterised by aggressive conditions, demanding the use of exotic materials, which can add substantially to the cost of the transmitter system. The linkages can also stick, fouling the measurement and requiring frequent maintenance. The overall accuracy of these devices is also often questionable — in some cases, accuracies are just 10% at best.

Capacitance probes comprise a long metallic probe, which normally enters the top of the separator vessel and extends to its lowest point. Liquid level and interface are detected by measuring the capacitance value between the wall of the vessel holding the liquid and the probe itself.

Again, the aggressive nature of most chemical and petrochemical applications will require the use of exotic materials, adding to the cost of the installation. Another complication is the measurement of sticky substances, which can coat the metal, resulting in measurement uncertainties and poor readings.

Other factors, such as foam on the liquid surface or vibration of the tank, can also conspire to reduce measurement certainty or even render the probe inoperable.

Differential pressure

Probably the best solution for measuring liquid interface levels is offered by remote seal differential pressure transmitters. With this technique, when the distance between the taps on the separation stack is filled only with the lighter liquid, the differential pressure is minimum value or the lowest range value (LRV) of the transmitter. When it is filled with the heavier liquid, the differential pressure is at its maximum value, or the upper range value (URV).

Although this technique overcomes many of the problems associated with the previously described methods, particularly with respect to corrosion, it does have one main drawback. The small difference in both the specific gravity of the two liquids and the distance between the taps on the separation stack results in a very small differential pressure span.

In many cases, the size of this span is often lower than the recommended minimum span for most remote seal transmitters. One way of overcoming this problem is to use remote seals and transmitters that are sensitive enough to detect very low span changes.

An example is ABB’s remote seal based 2600T interface level transmitter, which has been specifically designed for use at very low differential pressures. These transmitters use a remote seal with a highly sensitive diaphragm available with a range of fill fluids for a variety of applications.

Protection against leakage of the fill fluid is ensured by an all-welded construction, which offers a significantly longer service life than seals with a conventional gasket or thread construction, particularly in vacuum applications.

A typical example of this technique in action can be found in a chemical plant that wanted an interface level transmitter for use in a chemically aggressive hydrocarbon reprocessing application. In this application, a mixture of process hydrocarbons cleaned from the plant’s tanks and reactors, and water used for cleaning the reactors, was piped into a holding tank where it was allowed to settle.

The customer wanted to be able to pump the hydrocarbon back into the process for reclamation without also pumping any of the water. In designing a solution, several obstacles had to be overcome. Firstly, the application involved a very low differential pressure span, impossible for most remote seal transmitters to measure. A second challenge was the location of the application, which was subject to considerable swings in ambient temperature.

Such inconsistent conditions can often pose a potential problem when measuring very small pressure differentials. To solve this problem, the entire transmitter, with remote seals connected, would have to be temperature characterised together in an environmental chamber.

A microprocessor-based ABB 2600T draft range differential pressure transmitter was installed because of its small upper range limit, suitable for the close requirements of the application. The temperature characterisation data from the environmental chamber was stored in the transmitter’s memory. The transmitter’s on-board temperature sensors monitor the ambient temperature. Accurate pressure measurement is ensured by the transmitter’s microprocessor, which compares the data from the environmental chamber with the ambient temperature conditions and adjusts the transmitter’s output accordingly.

A major concern at the outset was the risk of any pressure imbalance inside the capillary system due to changes in ambient temperature, which would cause the fill fluid to expand or contract. The effect of this potential change was calculated under laboratory conditions, with the uncertainty being predicted at less than 0.5% of span.

Since this new transmitter was installed, the interface level control has greatly improved. The customer has also reported that downtime has been eliminated, saving over £30 000 per year on the cost of maintenance alone. Before this, monthly maintenance was required to clean the previously installed buoyancy transmitter system to prevent shutdowns.

Trevor Dunger is an ABB product specialist.