Pumping high viscous fluids for plastic pellet manufacturing

Megaroyal Polymer
Photo: The Megaroyal triplex monobloc pump: engineered for complex critical fluids

NFM / Welding Engineers Inc., a leader and manufacturer of Extrusion Systems for the rubber and plastics industries, asked Milton Roy to develop a solution for an end-user located in China to pump and meter a highly viscous polymer product. Milton Roy responded by customizing the Megaroyal® pump to fit seamlessly into the user's application.

Headquartered in Ohio (USA), NFM Welding Engineers can deliver complete turnkey extrusion systems to organizations operating in the plastic pellet production industry. In an effort to fulfil a request from one of its clients in China, the engineering company enlisted Milton Roy's expertise to incorporate a pump into the client's process that would be capable of pumping an especially viscous polymer mixture solution. Liem Chau, Global Product Manager at Milton Roy, explains: "We had to come up with a solution featuring enough power to inject an extremely viscous product (over 10,000 centipoise) at high temperatures." Manufacturers need to maintain elevated production rates to keep pace with the rocketing level of demand in Asia's plastic pellet market, meaning that they need efficient pumps offering high flow rates.

Photo: Liem Chau, Global Product Manager at Milton Roy, standing next to a Megaroyal pump during the testing operation.


The client's process included a polymer solution fed from a pressurized tank and heated to 194°F (90°C). Pumps are needed to inject the product through a preheating system and into a buffer tank, followed by the extrusion machine. The product is then water-cooled and cut into pellets by a rotating razor-sharp blade. For this specific application, Milton Roy proposed the Megaroyal triplex monobloc pump, which is frequently used in the oil & gas and chemical industries due to its compliance with the API 675 standard. "In 2019, Milton Roy supplied four Megaroyal pumps to this production site," advises Liem Chau. "The pumps are required to operate continuously in this mission-critical process, where the slightest production shutdown would be a deal-breaker. The insulated pump meets the customer’s requirement by offering a high flow rate of 2245 GPH (8.5 m3/h), while withstanding variations in discharge pressure."


The double-diaphragm PTFE metering technology built into the Megaroyal pump is the same technology used in the Primeroyal® pump series for over 20 years, which has given Milton Roy real expertise and insights into the metering process. The Megaroyal pump can deliver flow rates up to 220 GPM (50 m3/h) and withstand discharge pressures up to 3655 psi (252 bar). End-users are looking to gear production towards demand, which explains why they want to achieve superior flexibility using a variable speed motor that can reduce production times by a ratio 1:5. The Megaroyal pump is available with three different liquid-end sizes.

Gaetan SPUDE  CFD2
Photo: Gaëtan Spude, R&D Manager, presenting the friction losses in the check valve, due to a CFD (Computational Fluid Dynamics) calculation


The Megaroyal pump is fitted with an internal refill valve that automatically controls the hydraulic oil to guarantee a consistent and accurate flow rate. This system also ensures reliable performance, even in the case of a blocked suction valve. "Our pumps incorporate an internal pressure relief valve to protect the pump and are equipped with a hydraulically piloted valve that protects the diaphragm from rupture,” explains Liem Chau. We also use a double-diaphragm leak detection system. In the unlikely event of a diaphragm leak, a pressure sensor detects a pressure increase and sends a warning signal to the control room. The pump will continue operating until the end of the production batch, even with a damaged diaphragm." The Megaroyal pump's double-diaphragm remains hermetically sealed to prevent the possibility of external leaks. That is why this model is perfectly suited to toxic, hazardous and radioactive products.


The fluid that needs pumping is especially critical to the end user. The high viscosity fluid combined with the need to maintain a specific temperature adds another layer of complexity to the process. If the liquid cools down, viscosity will increase and the pump won’t deliver the correct flow rate, with the risk of a bottleneck in the check valves. Since the product is extremely viscous, the cross-section of the area and the check valve sizes were increased to reduce any friction loss and ensure cavitation-free pumping. Gaëtan Spude, R&D Manager at Milton Roy, explains: "With a high vapor pressure of 213 psi (14.7 bar) and a minimum suction pressure of 246.5 psi (17 bar), the gap between the NPSHa and the NPSHr, i.e., between the suction pressure available in the system and the suction pressure required for the pump, is fairly low. The viscosity rapidly causes friction loss, so the cross-sectional area must be increased." On the other hand, it is important to factor in the fluid's compressibility. Oversizing the cross-sectional area and the volume of fluid in the metering pump would quickly lose the flow rate. Gaëtan Spude continues: "If a fluid is compressed by 10%, the pressure loss will be much higher than 10%. The volume subjected to the positive/negative pressure effect is much greater than the pump's capacity, which acts as a multiplier. Therefore, we had to strike the right trade-off between avoiding oversized cross-sectional areas to prevent a fall in flow rate and having large enough cross-sectional areas to prevent cavitation in the pump. To achieve this goal, we carried out CFD (Computational Fluid Dynamics) calculations." At the same time, the Megaroyal pump limited friction losses in the piping due to a phase shift between the three plungers of a triplex pump. Milton Roy was faced with the challenge of reconciling compressibility with viscosity.


One of Milton Roy's channel partners, Industrial Pump & Equipment Corp. (IPEC) and sales engineers in the USA worked with NFM Welding Engineers to adapt the pump based on a standard Megaroyal model. The end user was delighted with the solution. NFM Welding Engineers is currently working on a new production line with the same Megaroyal model of pumps. "The four pumps have performed well. The first installation also allowed us to check the accuracy of our calculations on-site," says Gaëtan Spude. "The field results showed that we had met the required flow rates. We proposed a pump with a speed of 117 strokes per minute and were expecting to achieve the targeted flow rate with 94 strokes per minute, and that is exactly what happened."