Diagnostics are trumps
Metering ethylene oxide via a process diaphragm pump combined with a monitoring system

Engineers at the Sasol plant in Marl have noticed the pump monitoring for the newly installed process diaphragm pump calculates the flow just as accurate as the originally installed mass-flow meter. This combination boosts operational safety in a critical step of production as well as cost effectiveness.
Ethylene oxide is a crucial intermediate for the production of ethylene glycol and other chemicals. One of the producers in Germany is the international operating gas-, oil- and chemical company Sasol. Ethylene oxide has a flashpoint of -57 C and its ignition temperature is about 435 C. Furthermore the gas converts together with air into an explosive mixture. Therefore most of the equipment at Sasol‘s ethylene oxide plant at Marl must be emission free. This naturally effects the pump selection. Along with standard centrifugal pumps a variety of magnetically coupled and can-type motor pumps as well as process diaphragm pumps are being used. Dr. Michael Seedorf, the leading operations engineer, uses leak-free pumps not only because of the emission free operation but also because no mechanical seals have to be maintained. Therefore less maintenance is required for this equipment. Together with his 14 colleagues, Seedorf is responsible for the technical equipment of a production unit. The main duties of this team are maintenance, selection of new equipment, and optimization of the production process as well as planning plant expansion.

Centrifugal pumps are still dominant, however, they have their limits, states Seedorf: multistage centrifugal pumps allow high operating pressures, but they are not suitable when it comes to handling precisely small flow rates at high pressure. In case the process requires small quantities of chemicals to be injected under high pressure, reciprocating pumps are the preferred selection. The Marl plant has installed a Bran+Luebbe Novaplex process diaphragm pump specifically for this purpose. The plant has a long record with pumps from this manufacturer: the process requires in one stage the precise metering of water and ethylene oxide. This was performed by a mechanically coupled double gearbox (Novados, vertical design). Process design, however, required the separation of these components.

While the existing pump still meters the water (after refurbishing), the ethylene oxide is handled by a new frequency-controlled Novaplex process diaphragm pump. An evident advantage is that the flow can be controlled separately. The RPM of the pump is controlled by the flow of the injected ethylene oxide and can be reduced, if necessary, to “0” flow (a characteristic that cannot be achieved with a frequency controlled centrifugal pump).

Ethylene oxide and its ratio is not the only essential in the process, but as it is toxic and highly flammable every measure has to be taken to avoid emission. Because of this it is indispensable to monitor the status of the pump and Sasol decided to order together with the new pump the on-line pump monitoring system Novalink-CSM.

Flow monitoring by the pump monitoring system

As the pump monitoring system allows the continuous calculation of the pump flow it is now possible to run a plausibility check for the built in mass-flow meter. This increases the safety substantially. Experience at Sasol shows that the flow calculated by the Novalink-CSM system proves to be practically identical with the flow measured by the built-in mass-flow meter.

Characteristically for an oscillating pump is that it delivers with each stroke an exactly reproducible volume of liquid. With taking the density into consideration adding up the number of strokes within a time range it is possible to calculate the mass-flow of the pump. This has proven to be possible in the Marl plant.

How does this Novaplex CSM pump monitoring system function? The hydraulic chamber of each diaphragm pumphead is equipped with a pressure sensor. This sensor measures the pressure development during a complete cycle of the crankshaft (suction and discharge stroke). An additional sensor monitors the RPM and the rotation angle at the end of the crankshaft. The integrated computer uses these signals to calculate in real-time a pV-diagram (pressure–stroke volume diagram) individually for each pumphead. These diagrams show all relevant characteristics that are necessary to deduct all functions for an optimal operation of the pump. The computer calculates at the same time a simulation model of the pV-diagram at the measured pressure for ideal pumpheads. Continuous comparison of the model and the real-time pV-diagram results in differences that are evaluated and used to indicate variations in pump status to the operator or to the automatic process control system. Alarms are indicated in case the pre-set limits are reached so that necessary action can be taken.

The main error conditions e.g. gas trapped in the product- or hydraulic-chamber, malfunction of the suction or discharge valves or pulsation dampeners are identified. The collected data also allow the calculation of volumetric efficiency, hydraulic power, and flow rates for each individual pumphead.

Conclusion: In case of operating non-critical process steps it may be possible to rely only on the flow calculation of the Novalink-CSM system instead of a separate mass-flow meter. Pumping ethylene oxide, however, requires both systems due to stringent safety regulations. The initially installed pump has shown very satisfactory operation for Sasol for two years and a second Novaplex process pump including a monitoring system is being installed.