Simply non-invasive
Clamp-on flow meter helps Shell optimize refinery filter system

The multi-stage filter system in the Hycon unit at the Shell Pernis refinery has a major impact on the quality and efficiency of the overall process. A clamp-on flow metering system from Flexim collects data which is used to optimize the system.
Frank Senteur
Shell Nederland Raffinaderij B.V. in Pernis and Europoort is one of the world’s largest refineries. It is number one in Europe with a capacity of 416,000 barrels of crude oil a day. This equates to 21 million tons a year or 44,000 liters per minute. In around 40 processing units, crude oil is converted into high-grade end products including gasoline, diesel, LPG, kerosene, naphtha and lubricant. Part of the remaining heavy oil fraction is used as an energy source to cover in-house needs, and some is sold as marine fuel. The key processes at the refinery are distillation, cracking (breaking down long oil molecules) and desulfurization. “We have come up with an ingenious hydrocracking process which enables us to convert heavy residuals into usable intermediates,” explained Robin Koevermans as he talked about the hydrocracking process at Shells’ huge complex at the Europoort site. Koevermans works as an engineering specialist at Shell’s Hycon unit. “We crack the heavy constituents, breaking them down into raw materials which can be used to make ‘normal’ products such as kerosene, gasoline and naphtha.” The Hycon unit can process a whole range of products and residuals, all of which are fed through a multi-stage filter to remove solid particulates that would otherwise degrade the process and contaminate the equipment including the catalytic converter. “Naturally, we want to reduce contamination to a minimum,” commented Koevermans.

The multi-stage filter

Koevermans went on to explain that Shell’s filter system is made up of four banks, each bank having ten filter cages for this vital filtering step. The system is primarily designed to remove iron and hard coke particles from the incoming raw material. Finding the optimal configuration of the filter system is a key issue, and Shell pays close attention to it. Differential pressure measurements are taken to assess the contamination level in filter banks A through D. When high differential pressure is detected at Bank A, the operators shut the bank down and clean the filter elements using flushing oil which is heated to 250 °C. Everything should be in tip top order again after the flushing operation is complete. However, differential pressure measurement is not sufficient to verify that this is in fact the case. Performance (flow) could theoretically be improved by using different filters which have, for example, a larger surface area, but a change of this nature would have to be backed up by reliable data. “We really wanted to know what is going on in the filter banks,” said Koevermans. “We knew what the total flow through the four banks was, but we did not understand exactly how the flow was distributed. Theoretically, distribution should have been uniform, but was that really the case? To find out, we wanted to measure the flow through each filter bank without having to buy four flow meters, and we also did not want to interrupt the process to take the measurements.”

Clamp-on flow measurement

How do you measure the flow through one filter without disconnecting the pipes? Joost van Parreeren, engineer and President of Flexim Benelux, knows the answer. “In many cases, it is easy to measure process parameters including pressure, temperature, level, flow and concentration from the outside wall of a pipe, vessel or reactor using clamp-on technology without the need to interfere with the process.” The advantages are obvious. First of all, there is no need to disconnect the pipe, so the process flow is not interrupted and there is no risk of causing new leaks. Clamp-on technology is safe (even in Atex zones,) and it does not cause any pressure or flow rate reduction, because the pipe remains fully intact. “Essentially, this measurement technology is just what users need, and the (petro) chemical industry is gradually embracing it,” claimed van Parreeren. “The technique is not only ideal for fixed installations. It is actually very versatile and is well suited for temporary applications. There is often no need to continuously measure flow or other parameters. Customers like Shell may only want to perform spot checks to keep track of inventory or detect certain trends.”

Practical application

Shell used a portable Flexim Fluxus ADM 6725 clamp-on ultrasonic flowmeter to measure the flow through all four filter banks. “It was very easy,” explained Koevermans. “All we had to do was strip back a bit of insulation on the pipe, mount the sensors, attach the meter and we were ready to go. You have to be careful, because the internal temperature in the pipe is very high (250 °C). The heat does not affect the flowmeter, however.”

Since the ultrasonic sensors are mounted on the outside wall of the pipe, they are not susceptible to mechanical wear caused by hot oil or particulates. “Of course we wanted to be sure that we could rely on the measurement results,” said Koevermans. “Measuring the flow of heavy oil is not the same as measuring flow in a water pipe.” By default, viscosity data for some of the substances which are commonly found in the (petro)chemical industry including water, acetone, glycol, gasoline, ammonia, different types of crude oil, etc. is stored in the Fluxus transducer. “However, heavy oil is different,” continued Koevermans. “We know its viscosity and density, so we entered these parameters into the flow meter controller ourselves.” That was no problem. Once the ultrasound sensors are installed on a filter bank, Shell engineers monitor flow behavior in the filter bank for an extended period of time. The transducer automatically stores the results in an internal data logger, working in unattended mode. The meter can be removed once data collection is complete. The data is then transferred over a serial cable to the hard drive on a PC, where it is imported into an Excel table. The data can be used to generate graphs and tables and to create a visualization of flow behavior and above all of what is going on in the filter banks. “We installed filter cages with large surface areas in the hope of increasing performance. However, flow measurement results showed us that the new, expensive filters were no better than the old ones,” explained Koevermans. “We reverted to the old filters. There is no point in spending more money on something that is not really any better. We will now take measurements at other points, and we expect that selective data collection will help us optimize performance and make the transition from regular maintenance to condition-based maintenance, which will reduce our costs.”

Reliable results

Flexim’s ultrasonic clamp-on flow meters are based on the familiar principle of transit time difference correlation. Good propagation of ultrasonic signals into the pipe and back has a crucial influence on the reliability of this non-invasive acoustic technique. Paste is normally used to ensure good coupling between the pipe and the sensor. However, depending on conditions at the mounting location, the paste has to be reapplied more or less regularly. The paste dries out at high temperatures, or wind and weather can wash it away.

Flexim has been using coupling pads and coupling foil for some time to eliminate this problem. Various foil materials can be used depending on media temperature. In combination with the patented WaveInjector, the flow meter can handle temperatures up to +400 °C, and it is also suitable for cryogenic applications. Flexim flow meters are very versatile and are compatible with nearly any medium including water, waste water, demineralized water, glycol, liquid ammonia (e.g. for cooling systems), acids, alkalis, alcohol and syrup as well as hydraulic and thermal oil. Non-invasive ultrasonic technology can be used in product detection and concentration measurement applications, and it is also suitable for
simultaneous flow and concentration measurement. The horizon of clamp-on ultrasonic technology has recently been extended beyond liquid flow measurement to include gas applications.n