Centrifugal Pump Spillback Policy

Table of Contents

1. Purpose

This procedure establishes Inflection Point Engineering’s philosophy for determining when a spillback system is required for a centrifugal process pump. Once a spillback requirement is established, this procedure also provides guidance for selecting the correct type of spillback system.

2. Definitions

2.1 Minimum Continuous Thermal Flow

The minimum continuous thermal flow is the lowest flow which the pump may operate without its operation being impaired by the temperature rise of the pumped liquid. The Hydraulic Institute defines the minimum continuous thermal flow as the flow where the temperature rise thru the pump reaches 15˚F.

The minimum continuous flow for high horsepower multistage, or high speed applications are more likely to be governed by minimum continuous thermal flow.

Note: In a Inflection Point Engineering Phenol complex, it may be necessary to specify a lower pumped fluid temperature rise limit than the Hydraulic Institute imposes for process reasons.

For example: Cumene Hydroperoxide (CHP) is thermally unstable and at elevated temperatures can initiate a self-heating reaction.

2.2 Minimum Continuous Stable Flow

The minimum continuous stable flow is the lowest flow at which the pump may operate without exceeding vibration limits imposed by API Standard 610. This flow is based on the hydraulic balance at the impeller. If the minimum continuous flow through a pump is insufficient, the resultant pump damage takes place over the long term and usually results in bearing or seal failure.

3. How to Determine Whether a Pump Spillback is Required

3.1 Is a Pump Spillback Required for Pump Protection?

• Does any operating case flow rate fall below the estimated pump minimum continuous flow?
• The Design Engineer shall compare the turndown, normal, and rated process flow rates for each process simulation case against the estimated pump minimum flow from NHP or T-501-03.

The Design Engineer shall refer to NHP summary report for insight into whether a spillback system is required for pump protection. The estimated pump’s minimum continuous flow is reported on this pump summary report and a warning message is provided to alert the design engineer that a pump spillback has been added.

For those pumping services which fall outside the range of the pump selection chart contact a rotating equipment specialist for a pump selection to define estimated minimum continuous flow.

Recognize some process services (Overhead or bottoms service) may have “soft” turndown requirements. In these services, the reflux or bottoms circuit may be able to be run at a higher flow, allowing the net overhead or net bottoms flow to turndown to 50 or 60%. See Table 5.1 for reference.

In general, err on the side of including pump spillbacks in the design recognizing the EPC contractor will have the opportunity and incentive to remove an unnecessary spillback from the design.

When requested by a customer to remove a spillback, obtain a specific pump selection from the pump specialist to try and improve the estimated minimum continuous flow.

3.2 Is a Spillback System required for Process Reasons?

The Design Engineer shall determine whether a spillback system is required for process reasons.

This may be done by:

• Reviewing the applicable Inflection Point Engineering P&ID module for mature/established Inflection Point Engineering processes
• Consultation with the appropriate Inflection Point Engineering OTS Field Service representative

Centrifugal pump spillback systems may be required to support off-design operation requirements, particularly at start-up. Some examples include:

• The Raffinate Pump on a Sulfolane Unit has a flow controller on the pump discharge and a spillback system with a control valve downstream of the Raffinate Cooler. This provides a constant hydrocarbon velocity through the Raffinate Water Wash Column.
• The Heavy Aromatics Pump on an Aromatics Fractionation Unit has a flow controller on the pump discharge and a spillback system from a downstream water-cooled exchanger through a control valve to the pump suction. This provides a low inlet temperature to the cooler which is usually too small to be an air-cooled exchanger. Once-through flow would exceed the recommended inlet temperatures for water cooled exchangers.
• The Clay Tower Charge Pump on an Aromatics Fractionation Unit has a flow controller on the pump discharge, and a spillback line with a control valve back to the Clay Tower Charge Tank. This provides a constant pressure for other aromatics streams coming from other units on level control in the complex.
• Pumps spillbacks may also be required to flush piping, inventory equipment, line out fractionation towers, etc.

3.3 Can the Pump be Blocked-in?

Recognize situations where a centrifugal pump may operate blocked-in.

• a. Can the pump be blocked-in?

Can the pump operate at a reduced flow rate by the failure action of a downstream control valve or other downstream operation (cycling of filters, driers, lead/lag reactors, etc.)?

Note: Inflection Point Engineering’s design philosophy does not add pump spillback protection to protect against inadvertent closing of the discharge isolation block valves on the pump.

b. Pumps operating in series:

When two pumps are operating in series, if either the low pressure or the high pressure booster pump fails, the remaining pump will operate at zero flow!

c. Pumps operating in parallel:

When pump are operating in parallel, if the internal clearances in one pump deteriorate substantially more than the other, the stronger pump may force the other to operate below its minimum continuous capacity!

d. Start-up of standby pump:

When a pump needs to be removed from service for maintenance, it is desirable to bring the standby pump on-line first to minimize the disruption to the process. However, this can result in both pumps operating below the estimated minimum flow for a short-period. Therefore, multi-stage pumps shall have a maintenance spillback installed when the discharge pressure exceeds 1000 psig (70 kg/cm2g).

3.4 Review the Pump Control Method

Recognize a process stream controlled strictly by level is likely to have a wider variation in flow relative to a stream controlled by flow.

Level controllers have little to no integral action, and therefore, will have greater error between set point and actual value for a longer duration. Also, a level controller only can result in the pump operating below minimum continuous capacity for a longer period of time in a situation when the destination pressure increases (pressurized storage).

The latest Inflection Point Engineering instrument and controls philosophy specify level cascaded flow controls when these situations can exist.

3.5 Review consequence to the process, plant personnel, or equipment if the pump were to operate below minimum flow

Single stage, overhung process pumps with typically low power requirements are less susceptible to damage and cheaper to repair when subjected to above situations.

Multi-stage (> 2 stages) are much more susceptible to damage when subjected to these situations and are much more costly to repair.

Therefore, multistage pumps with a differential pressure of 500 psi (35 kg/cm2) or greater, or a horsepower of 300 (225 kW) or greater, require a spillback system AND an automatic low flow shutdown of the driver.

The automatic low flow shutdown removes energy input in the upset event and limits the duration / severity of dry running condition when the suction vessel liquid level is lost.

Most Sundyne pumps require spillback systems because of their limited turndown capability, high temperature rise, and drooping head curves. Specify a spillback anytime the pump turndown requirement is 60% of the rated pump capacity.

3.6 Review the frequency, duration and / or severity of operation below the estimated pump minimum flow.

Advise the pump specialist how often, how long, and the magnitude of any expected operating excursion below the estimated pump minimum flow.

3.7 The Degree of Variability of the Process Conditions

Some Inflection Point Engineering processes are inherently more dynamic than others. Some examples of process units where operating flexibility is paramount include:

• Crude Unit: Variability in type of feed crude processed
• Vacuum Unit: Variability in type of feed
• FCC Main Column or Gas Concentration Unit may operate in Max. Gasoline or Max. Propylene modes.

Expect to use a greater number of pump spillbacks in these units so as not to limit refiner operating flexibility. This can also lead to an overall reduction in energy consumption.

4. Spillback System Selection

4.1 Types of Spillback Systems

The Design Engineer must choose one of the three types of spillback systems when a pump spillback is required. The three types are presented below in ascending order of cost and complexity.

a. Type A

A “Type A” spillback (see Attachment 1) represents the lowest capital cost spillback solution and consists of a spillback line with gate valves and a restriction orifice. The contractor shall size the orifice based on the pump vendor defined minimum continuous flow rate.

The Type A spillback system is most often applied to small pumping services where the operating costs of continuously spilling back the pump minimum continuous flow do not exceed ~$1500/yr. This is based on a 3 year payback to install a Type C spillback control valve (~ $4500).

Rule of thumb: Unless the extra energy associated with increasing the flow thru the pump continuously with a Type A spillback is < 3.5 hp (2.5 kW), specify a Type C spillback valve.

The Design Engineer may calculate the differential operating cost using:

Where:

A.O.C.Type A = Annual operating cost, $/yr for Type A spillback

A.O.C. Type C = Annual operating cost, $/yr for Type C spillback

The annual operating cost can be calculated using:

Where:

A.O.C = Annual operating cost, $/yr

kW = Pump power requirement, kW

t = number of operating hrs./yr. (Normally 8500 hrs or 97% availability)

Electrical Cost = Site Electrical cost, $/kW-hr

The pump power requirement with continuous spillback and without may be calculated using:

Where:

kW = Pump shaft power, kW

QType A = Est. Pump Min. Flow + Net Forward flow, gpm

QType C = Est. Pump Min. Flow

ΔP = Pump differential pressure, psi

η = pump efficiency

The Design Engineer shall consider the improvement in pump operating efficiency for small centrifugal pump services since it increases almost proportionally to capacity for the smallest pump frame size when operating at or very near the pump’s minimum continuous flow.

Note: The pump efficiency is only available on the pump vendor’s performance curve.

The Type A spillback systems may also be provided to cover infrequent, transient operations. For example: Start-up circulation, chemical treating, vessel filling, catalyst regeneration.

b. Type B

A “Type B” spillback (See Attachment 1) consists of a spillback line with a globe valve and a local FI in the pump discharge.

Type B spillbacks are typically limited to infrequent, transient type operations. (For example: Establish start-up circulation, chemical treating, vessel filling, catalyst regeneration, etc.)

c. Type C

A “Type C” spillback (See Attachment 1) consists of spillback line with a control valve automatically controlled by a board mounted FIC in the pump discharge. For pumps with branched discharge lines the FIC shall be in the total pump discharge line.

Attachment 1 shows P&ID representations of these three systems.

Attachment 1 Types of Spillback Systems

Type A	Type B
Type C	Type C

5. Modeling Spillbacks in Hydraulics

The Table 5.1 below is a reference for instructions on how to properly model some of the most common spillback situations which occur in Inflection Point Engineering process designs in both NHP or P9.8 hydraulics.

Spillback Scenario:	Procedure
1. Flow at turndown < est. pump min. cont. flow	” OR ”
2. Overhead pump where spillback can be avoided if the reflux flow is increased at turndown	IPE-TM-510-12 Section 5 “Typical Pump Spillback Hydraulics P9.8” OR IPE-TM-510-13 Section 5 “Typical Pump Spillback Hydraulics NHP”
3. Normal flow < est. pump min. cont. flow TYPE C spillback	OR
4. Normal flow < est. pump min. cont. flow TYPE A spillback	IPE-TM-510-14 Section 5 “Non-Typical Pump Spillback Hydraulics P9.8” OR IPE-TM-510-15 Section 5 “Non-Typical Pump Spillback Hydraulics NHP”
5. Normal flow for lowest process flow case < est. pump min. cont. flow BUT normal flow for other case > est. pump min. cont. flow	IPE-TM-510-14 Section 6 “Non-Typical Pump Spillback Hydraulics P9.8”
6. Normal flow for lowest process flow case < est. pump min. cont. flow BUT normal flow for other case > est. pump min. cont. flow	IPE-TM-510-15 Section 7 “Non-Typical Pump Spillback Hydraulics NHP”

6. Destination of Spillback Line

It is preferable to send the spillback line to a point on the suction side of the pump that allows the heat rise from pumping to dissipate prior to being reintroduced to the pump suction. Typically this destination is the vessel from which the pump is taking liquid. Determine the temperature rise of the fluid through the pump using:

where:

Cp = fluid specific heat of the fluid, Btu/lb-F

TDH = Total pump differential head, ft

η = pump efficiency at the given capacity

The temperature rise of a blocked-in pump can be determined using:

where:

Pshutoff = Pump shutoff pressure, psig

m1 = mass of liquid in pump casing

cp = specific heat of the fluid, BTU/lb-°F

Sometimes, for high horsepower and high head (multistage and Sundyne) pumps, it is necessary to route the spillback through a cooler unless the fluid is adequately sub-cooled. In some instances the pump is far away from a suction vessel or cooler. In such cases, if the head and horsepower of the pump are not too high, connect the spillback line directly to the suction piping. The Pump Specialist must review this situation. Show the connection on the P&ID to be at least 20 feet (6000 mm) upstream of the pump.

7. Customer Deviation Requests

Sometimes a contractor or owner will request the requirement for a spillback system be deleted for a particular service shown in Project Specification 501. This request is based on the actual pump purchased.

Before accepting the deviation request, it is imperative the Design Engineer understands why the spillback system is required. Obviously, the request may not be allowed if the spillback system is provided solely for process reasons.

Otherwise, review the request for waiver on its merits.

8. Automatic Recirculation Control Valve Technology:

As an alternate spillback system arrangement, Inflection Point Engineering is also aware of the Yarway Automatic Recirculation Control Valve technology. While Inflection Point Engineering has not standardized the use of this device on any specific services to date, Inflection Point Engineering would consider their use based on a specific customer request. Some key issues that need to be addressed when considering these devices are:

Hydraulics must reflect the higher pressure drops associated with these devices (5-12 psi depending on ARC valve sizing vs. approximately 1 psi for conventional flow measurement device and check valve)

Restricting the use of the ARC valve technology to easier, lighter duty services (i.e. clean, cool fluids, in moderate pressure applications). However, limiting ARC valves to these types of services makes it more difficult to justify economically.

Consult the Pump Specialist regarding any application (service, sizes, experience, etc.) of the ARC valve technology.