## 1 Introduction  
One of the most commonly used turbomachines in chemical and petroleum industries is the multistage centrifugal pumpp. In the construction of potential infrastructure for chemical processing, multistage centrifugal pumps still play an important role. They are usually employed for high-head applications, such as in boilers and power plants. They are capable of handling low-viscous, volatile and flammable fluids and can operate in temperature ranges from \(-120\)\({}^{\circ}\)C to \(370\)\({}^{\circ}\)C [1]. Here, low-viscous fluids refer to fluids having viscosities lower than normal water. Compared to single-stage pumps, the efficiency, reliability and performance of these pumps are substantially more important because of the extreme operating conditions. Consequently, it is necessary to be aware of the turbulent fluid flow behavior inside the pump in the design stage itself. This can be achieved by careful consideration of all the stages in the planning, design and testing in both design and off-design conditions. Manufacturers turn to computational methods to test design features, decrease the number of experimental tests conducted and identify undesirable design features at an early stage to save resources and time. Computational fluid dynamics (CFD) tools are used by turbomachinery designers to design pumps that function more efficiently. Modern pumps are first designed theoretically, tested using CFD tools and then by laboratory experiments. The laboratory-tested pumps are again studied using CFD simulations, and optimization strategies are applied for improving the pump's performance before manufacturing. [PERSON] et. al. [2; 3; 4] used CFD simulations to design, test and optimize centrifugal pumps for pumping multiple fluids used in the oil and gas industries. [PERSON] et al. [5; 6; 7] tested centrifugal pumps used for pumping water and light crude oil through experimental analysis and optimizing them using CFD tools. [PERSON] et al. [8; 9] studied the forces in a three-stage centrifugal pump at design and off-design conditions using CFD simulations. Pumps are designed to withstand high temperatures and pressures, and they are able to pump fluids of different densities and viscosities. Several multistage centrifugal pump designs are available on the market for the transportation of chemicals [10; 11], although a new pump design is required for pumping perilous chemicals with high head and high flow rate requirements in petrochemical refineries. The proposed pump design consists of a double-suction impeller and a twin volute at the first stage. A single-suction impeller with a diffuser and return vanes are added to Stages 2-4. Stage 5 consists of an impeller followed by diffuser blades and an outlet volute. A double-suction impeller design is chosen for the first stage due to the back-to-back arrangement of the two impellers. This arrangement provides a zero average axial force [12]. Double-suction impellers are used in centrifugal pumps for obtaining large flow rates, improving suction performance, reducing axial thrust on the bearings and cutting down capital cost [13]. The fluid enters independently into the two suctions and flows into the impellers. The fluid leaves the impellers together into the volute. Although the axial force of the double-suction impeller is theoretically balanced, it is not possible to overlook the radial forces. Two strategies can be used to decrease the impeller radial force of the double-suction stage. One is using a staggered impeller that has been studied previously [14; 15] while the other is to use a twin-volute casing. [PERSON] et al. [16] studied the effects of a twin volute in a submersible centrifugal pump and obtained a uniform distribution of high absolute velocity in the volute, homogeneously distributed flow structures and a flat high-velocity profile at the volute throat. [PERSON] et al. [17] used a triple-volute casing design to reduce the radial thrusts in a single-stage centrifugal pump.