A. Dominic, ADNOC Refining, Abu Dhabi, United Arab
Centrifugal pumps are used for a wide range of
applications and services in the refining industry. In today’s dynamic
scenario, changes in pumped fluid characteristics are common, especially for
transfer/loading pumps. For the changing process fluid parameter, pump performance
should be predicted to analyze the suitability of the pumping system for the proposed
service conditions. A pump originally in service for the bulk loading of fuel
oil was assessed for its suitability in use with higher viscosity vacuum residue.
Using the existing pump for a new product opened a new business opportunity
without any major investment. The original pump was installed in the early 1980s
and has been in service since.
Centrifugal pump performance curves are
provided by the original equipment manufacturer (OEM) for specific fluids with
characteristics like density and viscosity at a defined pumping temperature. Pumps
are performance tested using water, and the test data is utilized to predict
the performance with the service fluid. When handling viscous fluid, the
performance of the pump will differ from the water performance—this change will be more
pronounced when handling fluids of higher viscosities. As the viscosity
increases, the hydraulic losses within the pump increase, leading to a reduction
in the head, flow and efficiency, and an increase in the power and NPSH3 (net
positive suction head required resulting in a 3% loss of total head at the
first-stage impeller due to cavitation).
The effects of viscosity on performance are
well explained in the ANSI/Hydraulic Institute Standard 22.214.171.124 The
standard clearly explains how performance parameters like flow, head,
efficiency and power can be predicted with the viscous fluid when water
performance values are known. The standard also captures the process to be
adopted for preliminary selection of a new pump for given head, rate of flow
and viscosity conditions.
Simply, viscosity is the property of a fluid
indicating its resistance to flow when an external force acts on it. The
resistance also varies with the temperature. For pumps, the viscosity of the
fluid handled at pumping temperature is the important measurement. When considering
viscosity, two terms—dynamic
viscosity and kinematic viscosity—are prevalent.
Dynamic viscosity (absolute viscosity) is a
measure of the fluid’s resistance to shear. The SI unit is pascal-second (Pa·s)
and the centimeter/gram/sec (CGS) unit is poise (P); a commonly used unit
is centipoise (cP) (1 P = 100 cP) (Eq. 1):
1 Pa·s = 1 Ns/m2 = 1 Kg/(m/sec) = 103
cP (Eq. 1)
Kinematic viscosity of the fluid is the ratio
of dynamic viscosity to its density. The SI unit of kinematic viscosity is m2/sec
and the CGS unit is Stokes. The most commonly used unit for petroleum products
is centistokes (cSt) (Eq. 2):
1 m2/sec = 104 cSt (Eq. 2)
Kinematic viscosity (cSt) = centipoise/grams
The temperature of the pumped fluid also has an
impact on the kinematic viscosity. For petroleum liquids, the kinematic
viscosity reduces with an increase in temperature.
For a centrifugal pump, the change in viscosity
of the pumped fluid has an impact on the predicted performance of the pump (FIG. 1). Each
centrifugal pump has a head-to-flow performance curve rather than a straight
line. The curve is due to the varying losses in the pump at different flows.
The losses are a sum of disc friction, mechanical, leakage and hydraulic losses,
among which disc friction is the major component. Disc friction is directly
influenced by the viscosity of the pumped media.
The maximum viscosity a centrifugal pump can
handle is a bit subjective. Many references limit the use of centrifugal pumps to
a maximum of 330 cSt. The impeller geometry and pump size play a role on the
maximum viscosity that can be handled, also considering the torque and power limits
of the pump shaft. The negative impact on efficiency and head may make the use
of a suitable positive displacement pump beneficial to lifecycle cost.
Depending on the pump size and geometry, centrifugal pumps’ viscosity limits can
vary from 25 cP–700
cP normally. Another major consideration is the effect on the system curve from
the increased viscosity. As the viscosity of the pumped fluid rises, more pipe
friction will occur, and the system resistance curve also rises. This must be
considered along with the revised pump curve to ensure suitability of the
Background. A pump’s performance should
be predicted for a different service fluid (higher viscosity hydrocarbon). Performance
data is available for the originally selected hydrocarbon but the water test
data is unavailable. Utilizing the available instructions in HI Standard
9.6.7-2021, a reasonable estimate of pump performance has been developed and is
captured in this article. The HI Standard does not offer a direct procedure for
this extrapolation. However, the principles described for selecting a new pump
for the fluid and the conversion of water performance to fluid performance are
utilized to arrive at the revised performance.
This method is used to evaluate a pump used to
transfer hydrocarbons from a tank to a ship; the loading originally selected was
to handle fuel oil at a maximum viscosity of 125 cSt for an application to
handle vacuum residue at 600 cSt. A new product with significantly different
properties was required to be transferred using the same pump and existing line
up. The performance curve available for the original 125-cSt fluid and water test
data is unavailable. The expected performance with the 600-cSt VR is developed
using the curves available for 125-cSt fuel oil.
The existing pump curve is taken as the
starting point. Parameters are tabulated from the curve, as shown in TABLE 1, for the
originally rated fluid. The pump is used to transfer fuel oil with a viscosity
of 125 cSt and a specific gravity of 0.92 at 80°C.
The best efficiency point (BEP) flow is first identified from the curve as 1,700
m3/hr, then the corresponding values of head and efficiencies from
the curve are noted (FIG.
2). The values are also tabulated for 0.6, 0.8 and 1.2 times the BEP
The BEP point is taken as the starting point
for sizing an equivalent water pump using HI instruction 126.96.36.199.6. This instruction
is used for the preliminary selection of a pump for given head, flowrate and
viscosity conditions when the water performance is known. In the following case,
the pump is already in service and so the curve BEP flow is considered rather
than the pump rated flow to arrive at the water BEP conditions, as both are
corresponding values. The BEP flow can be seen to be 1,700 m3/hr at
a differential head of 135 m. Viscosity and specific gravity at a pumping
temperature of 80°C are 125 cSt and 0.92, respectively. The approximate water BEP
performance can be calculated with this data. Use the values to calculate
parameter B by applying Equation 10 of the HI Standard, shown here as
B = 2.8 x 1250.5/(1,7000.25
x 1350.125 ) = 2.64064
If 1.0 < B < 40, go to the next
step. As the value of B is falling in range, continue to the next step
of calculating CQ and CH.
The flow correction factor (CQ)
and head correction factor (CH) must be calculated for the
water BEP flow condition using Equation 4 of the HI Standard, shown here as Eq.
CQ = CH = (2.71)A = 0.989221
A = -0.165 x (log102.64064)3.15 (Eq. 4)
Using this correction factor, calculate the
water BEP flow and head (Eqs. 5 and 6):
Water BEP flow QW-BEP = Liq BEP flow/CQ = 1,700/0.989221 = 1,718.5 m3/hr (Eq. 5)
Water BEP head HW-BEP = Liq BEP head/CH
= 135/0.989221 = 136.47 m (Eq. 6)
For sizing a brand new pump, the above values of water head and
flow can be used to select a suitable pump. As the effort here is to predict
the performance of an existing pump, the following approach is used. Calculate an
efficiency correction factor using Equation
7 of the HI Standard, shown here as Eq. 7:
Cƞ = 2.64064^-(0.0547 x
2.64064^0.69) = 0.901406 (Eq. 7)
The correction factor for efficiency and flow remains the same for
all flows. The CH varies with flow and must be calculated for
each case using Equation 6 of the HI Standard, shown
here as Eq. 8:
Calculating for 80% of BEP (Eq. 8):
CH = 1 – (1– 0.98922)(0.8^0.75) = 0.991 (Eq. 8)
Water head at 80 % = liquid head at 80% BEP/CH at
80% BEP = 144/0.991 = 145.325 m
Water flow at 80% = liquid flow at 80% BEP/CQ at
80% BEP = 1,360/0.98922 = 1,374.82 m3/hr.
The calculation is extended for other flows—namely 120%, 60%, 40% and 20% of the BEP flow.
The calculated correction factors are applied to the entire range of
For this case, the water efficiencies were already available in
the performance curve but not for the service medium, so the same was used for
the water case. If liquid corrected values are available in the curve, the
water efficiency can be calculated with the same approach (TABLE 2).
Now that the water performance of the pump is established with a combination
of a 188.8.131.52.6 reverse iteration of the process defined in 184.108.40.206.5, the pump
performance for the new conditions can be established applying 220.127.116.11.5
straight away. The conditions for the new proposed liquid and the water
performance of the pump are the starting point for this step, as shown in TABLE 3.
Parameter B is calculated, taking into consideration the water
performance as per Equation 2 of the HI Standard, shown here as Eq. 9:
B = 16.5 x (6000.5 x 136.50.0625)/(1,718.50.375
x 1,5000.25) = 5.4186 (Eq. 9)
The 1.0 < 5.4186 < 40 calculates CQ as per
Eq. 4 of the HI Standard (shown here as Eq. 10) as valid for all flows. Once CQ is known,
the viscous flow can be calculated as Qvis = CQ
x QW (Eq. 11):
CQ = (2.71)–0.165 x (log10 B)3.15 (Eq. 10)
CQ = (2.71)–0.165 x (log10 5.4186)3.15 = 0.93982
Qvis = 0.93982 x 1,718.5 = 1,615.1 (Eq. 11)
At BEP flow, use the same value for CH. At
other flows, calculate CH using Equation 6 of the HI Standard, shown here as Eq. 12:
Cƞ can be calculated with
Equation 7 of the HI Standard (shown above as Eq. 7) and remains same for all
calculated values for the new conditions are tabulated in TABLE 4 using the
derived water performance values.
Note: The standard also gives charts for
correction factors for different B values and can also be used in place of the calculations.
The predicted HQ curve vs. the original service is shown in FIG. 3.
Takeaway. Though the method may
not predict the exact performance as in a performance test, it provides a
fairly accurate estimate and can be of great value. In an agile business intent
on maximizing revenue with minimum capital expenditure, a quick assessment of
utilizing existing assets can be of significance. The derived curve for the new
conditions can be used to evaluate potential limitations in pump discharge
pressure, NPSH margin and prime mover limitation while handling the proposed
fluid. The values are comparable with a predicted curve sourced from the OEM
and can assist in preliminary system assessments with reasonable accuracy. Field
observations of the available margins in the pumping system would be used to
evaluate the suitability of the pump for the revised case. HP
ARUN DOMINIC is a Mechanical Engineer with more than
25 yr of experience in rotating equipment in refineries. His experience covers
the entire lifecycle of assets from selection, commissioning, maintenance,
reliability and rerating. He now works as Team Leader of rotating equipment
engineering with ADNOC Refining in Abu Dhabi, UAE.