Centrifugal Pump NPSH Vessel Elevation Calcs

1. Table of Contents

2. Purpose

This procedure defines how to establish the correct suction vessel elevation when governed by centrifugal pump Net Positive Suction Head Requirements.

It also defines how to calculate available Net Positive Suction Head for centrifugal pumps when the suction vessel elevation is known or set by an alternate design criteria.

3. Definitions

NPSH is normally referred to in two ways:

• "NPSH Required" (NPSHR) is the head, in excess of the fluid vapor pressure, required at the suction flange of a specific pump to overcome the internal pressure drop in the pump suction nozzle and the passages to the eye of the impeller. The pump manufacturer determines the NPSH required. The NPSHR is a function of the pump/impeller design, and capacity.
• "NPSH Available" (NPSHA) is the net liquid head at the pump suction flange (Rated operating pump suction pressure – fluid vapor pressure). The NPSH Available must be equal to or greater than the pump NPSH Required in order to avoid pump cavitation.

Cavitation physically damages the pump and leads to loss of pump hydraulic performance and higher maintenance costs for refiner.

4. Select the Type of Pump

The pump type is selected by the New Hydraulics Program (NHP) in accordance with Inflection Point Engineering Tool ” to meet the required operating conditions.

Inflection Point Engineering recognizes there is overlap in hydraulic coverage between different pump types. Inflection Point Engineering’s rotating equipment group has made initial judgments based on experience for all operating conditions to maximize efficiency of the Inflection Point Engineering hydraulics/work process.

A common goal among design engineers during new unit design is minimizing the suction vessel elevation to minimize system erected cost. This is appropriate PROVIDED:

• It does not compromise the hydraulic fit of the centrifugal pump offered. API Std. 610 and Inflection Point Engineering strive to place the rated operating point within 80-110% of the pump best efficiency point (BEP).
• It does not lead to excessive deviation from the established Inflection Point Engineering work process without substantial value added. Some examples of where it is warranted are:
• The resultant receiver elevation would be too high to allow an upstream air cooled condensers to free drain into the receiver.
• The suction vessel is a major fractionation column in the refinery (Xylene Fractionator, Main Column, Vacuum Column, etc.).
• The total installed system cost is documented and actually reduced. Utilizing a half speed process pump in place of full speed pump increases the pump capital cost since the pump must be larger to handle the same flow at a reduce speed. Switching from a single stage, overhung process pump to a between bearing, double suction pump design results in a big step change in pump capital cost.

Refer to Procedure for additional background.

5. Inflection Point Engineering Design Philosophy for Establishing Suction Vessel Elevation when Governed by Centrifugal Pump NPSHR

The Inflection Point Engineering Design Engineer is required to design the system and determine the elevation of the suction vessel BEFORE details of the pipe layout and actual pump purchased are known.

The NPSHA of the system we design must exceed the pump NPSHR. The formula for calculating the elevation of suction vessels governed by centrifugal pump NPSHR is:

H = NPSHR + Hp + ΔHF - ΔHS - HL + HC

where H = minimum elevation of vessel above base line(ft)

NPSHR = pump NPSH required (ft)

Hp = pump centerline elevation above base line (ft)

HF = rated friction loss in suction piping (ft)

HS = system operating pressure minus liquid vapor

pressure (ft),

PS = system operating pressure (psia)

PVP = liquid vapor pressure (psia)

HL = minimum liquid level in suction

vessel (ft)

HC = NPSH design margin

NHP calculates the suction vessel elevation needed to satisfy the estimated pump NPSHR in this same manner. See Sections 5.1- 5.6 for more detailed description on each term.

Also, before setting the final vessel elevation, consider mechanical and physical requirements such as access clearance and bottom piping configuration. For example, in a Inflection Point Engineering Hycycle Process unit, the EHS Bottoms pump is installed vertically in the piping below the Enhanced Hot Separator. Additional vessel elevation is required to pull the back end of the pump out the bottom for maintenance.

5.1 Establishing Centrifugal Pump NPSHR

NHP uses the data in Tool to estimate, in feet of liquid, the minimum pump NPSH requirement. Select the pump based on RATED capacity and RATED total differential head (TDH).

T-501-03 was developed considering the capabilities of a cross-section of global supplies and is based on centrifugal pumps with maximum impeller suction specific speed (Nss) of 11,000 (English units). This is consistent with Inflection Point Engineering Standard Specification 5-11 Section 6.1 Paragraph 6.1.9. While this type of pump / impeller design has a higher relative NPSHR, it is consistent with refining industry practices and has proven to operate more reliably over a wider operating range.

NOTE: The NPSHR values reported in T-501-03 include an additional 2 feet to minimize requirement for shop testing of NPSHR. Shop NPSH tests add expense and extend delivery time. When a suction vessel elevation is considered too high, the Inflection Point Engineering Design Engineer may reduce the NPSHR reported in T-501-03 by 2 feet.

For some revamps, a new pump may have to be retrofit into an existing or partially existing system. In this case, it may be necessary to select pumps with a maximum impeller suction specific speed of 13,000. Contact a Rotating Equipment Specialist to make a specific pump selection or to discuss other solutions if there is insufficient NPSH available.

The NPSH Calc spreadsheet, T-501-04, is a tool to establish centrifugal pump suction vessel elevation outside of NHP. This is useful for circuits where hydraulics are calculated by hand, when Preliminary Equipment Data (PED) information is being developed for revamps, etc.

5.2 Establishing Pump Centerline Elevation

NHP assumes the pump centerline elevation as presented below:

Rated Flow	Pump Centerline Height
0 to 600 gpm	2.5 ft
601 to 1800 gpm	3.0 ft
1801 to 2500 gpm	3.5 ft
2501 to 8000 gpm	4.0 ft

The pump centerline elevation reported above includes a one foot concrete foundation projection above the process unit base line, plus the vertical distance from the pump base plate to the pump shaft centerline. Pumps are installed with a one foot foundation projection as protection against flooding.

5.3 Calculating Friction Loss, ΔHF

Inflection Point Engineering design philosophy calculates the expected rated suction pipe friction loss by multiplying the assumed equivalent length of pipe by the appropriate rated ΔPf/100 eq./ft for the given suction line size. For branched suction piping systems, calculate the friction loss for each separately and sum together.

Refer to Procedure for background and information on how pump suction piping equivalent length should be estimated. Section 5.6 addresses provisions for additional pressure drop in the suction strainer due to clogging.

For turbulent flow, the formula for correcting pressure drop is given below:

ΔPf / 100’ = ΔPo*sp grf*(vf/0.6)0.2

where ΔPf/ 100’ = pressure drop corrected for specific gravity and viscosity at flowing conditions

ΔPo/100’ = uncorrected pressure drop from Attachment 3

sp grf = specific gravity at flowing conditions

vf = kinematic viscosity at flowing conditions in centistokes

NHP sizes the pump suction line based on normal flow but calculates the rated suction piping friction loss based on rated flow and on clean pipe friction factors (Inflection Point Engineering default relative roughness, ε = 0.00015).

When handling significantly sub-cooled liquids, size the pump suction line based on a ΔPo/100 eq. ft = 0.35 psi/100 eq. ft., or a maximum velocity = 500 fpm, whichever is greater. The design engineer shall consider the degree of sub-cooling present and apply more stringent criteria when necessary.

Suction piping system pressure drop may significantly impact the elevation of the suction vessel. Recognize that increases in the suction piping size will reduce the pressure drop in the suction piping system and may reduce both the elevation and the overall cost of the suction vessel.

5.4 System Vapor Pressure Credit, ΔHs

The vapor pressure credit is the difference between the fluid vapor pressure at pump suction temperature and the suction vessel operating pressure, converted to feet of head. Most pumped fluids are considered “bubble point” fluids, or fluids where the liquid / vapor interface are at equilibrium. The vapor pressure credit for bubble point fluids is zero.

Consider any additional system pressure above the fluid vapor pressure to be NPSH available. This situation most likely exists in liquid full systems or in systems handling sub-cooled materials. Possibilities for a vapor pressure credit include:

• Systems that contain a cooler in the pump suction piping
• Pump from storage tanks.
• Sidedraw pumps
• Pump operating in series with another pump

Sub-cooled liquids in gas blanketed surge drums or receivers are special situations. Consult with the appropriate Technology Specialist for specific policy. In some processes, even though the liquid is sub-cooled, Inflection Point Engineering may consider the system design as if the liquid were at its bubble point to be conservative.

5.5 Minimum Liquid Level in Vessel, HL

The Inflection Point Engineering design philosophy is based on a minimum liquid level of 6 inches.

For horizontal vessels, the minimum liquid level is 6 inches above the bottom.

For vertical vessels, the minimum liquid level is 6 inches above the bottom tangent line.

Exception: Inflection Point Engineering Phenol Oxidation Section Oxidizers are large, grade mounted tanks and system NPSHA is based on normal liquid level.

5.6 NPSH Design Margin (Contingency), HC

Inflection Point Engineering design philosophy sets NPSH design margin, or contingency, equal to fifteen percent of the estimated pump NPSHR (from Section 5.1), or two feet, whichever is greater.

Possible exceptions:

Consider whether accelerated clogging of the pump suction strainer will occur in dirty services where coke or other materials may accumulate. If accelerated clogging is anticipated, add an additional contingency equal to the pressure drop that results from the equivalent of 250 pipe diameters of additional piping.

Consider additional piping loss if the suction piping has a tendency to become dirty or fouled in operation.

Consider preinvestment in the system design for future greater pump capacity due to a larger impeller / motor, but only when specifically requested by the customer in design basis or BEDQ.

Lastly, some clients impose alternate NPSH design margin guidelines on Inflection Point Engineering. The Shell DEP’s are the most commonly encountered alternative. These guidelines need to be implemented during hydraulics first pass to avoid costly rework. The Shell DEP NPSH design margin typically adds 1 meter contingency at 125% of rated capacity. With the correct manual inputs, these client requirements can be met. See a Rotating equipment specialist for help, if needed.

Calculations files shall clearly record the amount of, and the reasons for, the contingency provided.

6. Inflection Point Engineering Design Philosophy for Calculating NPSHA when Suction Vessel Elevation is Existing or Governed by Alternate Criteria

In a revamp the suction vessel elevation is fixed. The Inflection Point Engineering Design Engineer is required to calculate the NPSHA.

Also, for new units there are several other situations where the suction vessel elevation is fixed and the design engineer must calculate the NPSHA for a centrifugal pump. These include:

• Fractionation column elevation set by thermosiphon reboiler needs
• Receiver or suction vessel supplying liquid to two different pumping services

In any of the above situations, NHP will calculate the NPSHA given a suction vessel elevation using:

NPHSA = H - Hp - ΔHF + ΔHS + HL - HC

where NPSHA = System net positive suction head available (ft)

H = Suction vessel elevation above base line(ft)

Hp = pump centerline elevation above base line (ft)

HF = rated friction loss in suction piping (ft)

HS = system operating pressure minus liquid vapor

pressure (ft)

=

PS = system operating pressure (psia)

PVP = liquid vapor pressure (psia)

HL = minimum liquid level in suction

vessel (ft)

HC = NPSH design margin

Note: The minimum suction vessel elevation is 5 ft unless the suction vessel is a sump or a tank.

7. 501 Centrifugal Pump Specification

The Inflection Point Engineering 501 Centrifugal Pump project specification shall report the NPSHA, rated suction pressure and fluid vapor pressure.

Note: Inflection Point Engineering design philosophy does not report the contribution to NPSHA from NPSHR contingency in the 501 project specification although it is included in the system design.

8. Insufficient NPSH Availability Problems and Solutions

Occasionally, problems involving the availability of NPSH occur in design work. In such cases, consider the following methods of reducing the NPSH requirement or increasing the NPSH available:

• Raise the minimum controlled level in the suction vessel.
• Evaluate increasing the size of the pump suction piping.
• Run two pumps in parallel for the rated capacity.
• Use a slow speed or double suction pump, if applicable.

NPSH problems may also result from spillbacks to the pump suction. The spillback material is usually warmer than the normal pump suction liquid. Cooling the spillback liquid is recommended to avoid raising the resultant pump suction temperature and adversely affecting the fluid vapor pressure margin. Flashing will occur if sufficient NPSH is not available. Reference Section 6 of Procedure ,” for the destination of the spillback line.