Low shear pumps

This article gives an overview of a pump’s effect on the shearing of production fluids in the oil industry. The article further summarizes the current state of low shear pump research and developments. 

Introduction

Most of the world’s oil reservoirs produce oil together with water. The liquids are subjected to shear forces through the pumps and are sheared as they pass through pressure-reducing devices in the production line. The shear forces disperse one liquid into another. A common oil field emulsion is a dispersion of water droplets in oil. Formation of emulsions in the main separation process is a big concern for the operators. The higher the shear forces, the smaller the droplets of the dispersed phase, and hence the more stable the emulsion. The stability of the emulsion determines how long it takes to separate the phases. Due to this, high shear pumps are not desirable in processes where emulsions may appear.

Pumps are the third most used equipment type in the oil and gas processing industry, with only valves and pipes being more used. Choosing the right type of pump is very important. Pumps are selected based on pressure head, capacity, weight and size, process control and price. 

The pumps are used in different systems throughout the process plant. The oil-water mixtures are pumped during the main separation process, as well as in produced water treatment and in multiphase transport. Emulsification of the fluids in these processes will often have a negative impact. It reduces the efficiency of the downstream separation equipment and increases the pressure drop during the pipe transport due to the higher viscosity of the emulsions. Emulsion treating operations require additional capital and operational expenses. This may include use of auxiliary equipment for emulsion breaking, use of chemicals and energy for heating, and associated operational costs.

Fluid shearing

The ability of the pump to avoid shearing of the process fluid is the important parameter in the pump selection process. This is especially relevant when fluid emulsification is a possibility. 

The physics associated with droplet size development inside the pump are complex and probably require extensive numerical analyses to be predicted, if possible. However, it does seem plausible that two different mechanisms are present at the same time. These are turbulent droplet coalescence and droplet break-up. 

Hinze has described the shearing effect in turbulent flow by formulating the maximum sustainable droplet size that can exist at given flow and fluid conditions:

$d_{max}=\mathrm {We} _{crit}^{{^{3}}/{_{5}}}\cdot \left({\frac {\sigma }{\rho _{c}}}\right)^{{^{3}}/{_{5}}}\cdot \epsilon ^{{^{-2}}/{_{5}}},$

(1)

where

d_max = maximum droplet diameter, m

We_crit = critical Weber number, -

σ = interfacial tension between the oil and water phases, N/m 

ρ_c = density of continuous phase, kg/m³ 

ε = energy dissipation rate per unit mass, m²/s³ or W/kg

The mean energy dissipation rate per unit mass, ε, is the parameter that describes the intensity of the turbulence. Identifiable structures in a turbulent flow are called eddies. The turbulent flow consist of eddies of wide size range. As the kinetic energy cascades from large scale eddies down to smaller ones, energy dissipates into heat due to viscous forces. Energy dissipation rate is the parameter used to determine the amount of energy lost by the viscous forces in the turbulent flow. In order to reduce shear forces and the droplet break-up of the dispersed phase, the mean energy dissipation rate must be minimized according to Eq. 1. 

During the study of rotor/stator mixers, Padron proposed the following relationship for the average turbulent energy dissipation rate per unit mass:

$\epsilon =kN^{3}L^{2},$

(2)

where

k = proportionality constant, -

N = rotational speed, rpm

L = length of the impeller, m.

Padron then suggested that this relation for energy dissipation rate can be approximated for a single stage centrifugal pump as well.

Pump types with low shear characteristics   

There are several studies performed on shearing characteristics of pumps. The common set up to study this effect is to pump water with relatively low oil content. With low concentration of the dispersed phase the coalescence is negligible, and therefore does not affect the droplet-break up mechanism. It is possible to evaluate the pump’s shearing effect on the fluid flow by comparing the outlet droplet size distribution to the fixed inlet size of oil droplets.

Progressive cavity pumps

Flanigan et. al. tested seven different pumps, representing five pump types. The results showed that all pump types tested experienced consistent droplet break-up. The progressive cavity pumps showed least droplet shear. The testing of a twin lobe pump revealed that, at constant differential head, the degree of droplet break up decreases with increasing flow rate. This trend was seen for all the positive displacement pumps. Following offshore tests were performed on progressive cavity pumps. Results concluded that no significant droplet shearing was observed in the field conditions. Average oil droplet size on the outlet size of the pump were around 95% of the inlet droplet size.

Gear pumps

Zhang et al. obtained experimental results on fluid’s droplet size distribution under shear conditions caused by a gear pump. The main observation was that the higher shear intensity (higher RPM of the pump) led to the reduced outlet droplet size. It was observed that at pump speeds over a certain threshold, the droplet size distribution was not affected further. The authors’ explanation  was that the residence time of the droplets in the pump was no longer higher than the required break-up time for the given droplet size. Another practical observation was that higher dispersed phase concentration resulted in less droplet break-up in the gear pump.       

Single stage centrifugal pumps

Centrifugal pumps tend to shear process fluids significantly, especially at high speed. Different studies were performed in order to investigate the possibility of low shear operation for this type of pump. Following requirements were drawn: 

•     Constant flow operation minimizes the shear
•     Pump is of closed impeller design
•     Pump operation is near the maximum on the efficiency curve (at least 70%)
•     Maximum speed is 1800 rpm.

In addition, Shell’s guidelines for pump selection for produced water handling added the following centrifugal pump design criteria:

•     Large impeller diameter
•     Oversized discharge nozzle (slow discharge speed)
•     Maintain a limited pressure boost per stage (less than 50 psi)
•     Operate the pump with a low specific speed (Ns<700).

Morales et. al. in their experimental study of centrifugal pumps, measured the effect of different parameters on droplet-size distribution. Results showed that pump speed has the strongest effect: the higher the speed, the smaller the droplet size. Mixture flow rate and water cut variations in the experimental study were found to have weak influence on the droplet size distribution. It has to be noted that in this study high concentrations of oil were used, varying from 25% to 50% of total volume.   

Coalescing multistage centrifugal pumps

The development of a new coalescing multistage centrifugal pump was reported by Van Teeffelen. Tests were performed and compared to an eccentric screw pump and a single stage centrifugal pump. Results showed that the coalescing centrifugal pump consistently increased the average droplet size for various process conditions. The eccentric screw pump generally maintained the droplet size of inlet droplet or slightly increased it, while the single stage centrifugal pump consistently sheared the inlet oil droplets. It was also concluded that coalescing effect of the new pump was increased when 1) the oil concentration was increased and 2) average inlet droplet size was reduced.

In further studies of the coalescing centrifugal pump, Husveg et. al. demonstrated that the point of operation affects the degree of coalescence. Results showed that for given fluid properties, there is an optimal combination of flow rate and pumping pressure to promote coalescence. This was referred to as the optimal point of operation.    

Nocente performed CFD modelling of this coalescing multistage centrifugal pump. The model revealed that recurrent turbulent structures were found in the passage between the diffuser and the return vanes. Numerical particle tracking simulation showed that these structures have a retaining effect on the oil droplets. The retaining effect proved having a higher impact for the smaller droplets, increasing their residence time in the pump and therefore increasing the possibility of coalescence. 

Nomenclature

dmax	=	maximum droplet diameter, m
k	=	proportionality constant, -
L	=	length of the impeller, m
N	=	rotational speed, rpm
Wecrit	=	critical Weber number, -
ε	=	energy dissipation rate per unit mass, m2/s3 or W/kg
ρc	=	density of continuous phase, kg/m3
σ	=	interfacial tension between the oil and water phases, N/m