Tower Internals Design Change Requests: Cost, Schedule and Hydraulic Impacts

Technical Guide | Tower Internals Engineering | KEMVORI Petrochemical Equipment Limited

Direct Engineering Answer

A tower internals design change should be evaluated for its effects on process performance, hydraulic calculations, mechanical interfaces, drawings, materials, procurement and project schedule.

Even a seemingly small modification, such as changing a distributor outlet diameter, relocating a nozzle or adjusting an equipment elevation, can affect the original engineering basis.

A design change should therefore never be treated as a simple drawing correction without first evaluating its technical and commercial consequences.

A controlled engineering change process should identify:

The original approved design.

The exact requested modification.

The reason for the change.

Affected hydraulic calculations.

Mechanical and installation consequences.

Material and fabrication status.

Cost and schedule implications.

Required technical and commercial approvals.

The objective is to ensure that every approved modification is properly evaluated, documented and implemented before the affected equipment proceeds to manufacturing or installation.

Why Tower Internals Design Changes Create Project Risks

Tower internals projects involve several interconnected engineering activities.

Process conditions determine hydraulic requirements. Hydraulic calculations influence equipment geometry. Mechanical drawings define supports, connections and installation arrangements. Manufacturing drawings translate the approved design into fabricated components.

A change in one area can affect several others.

For example, an EPC contractor may request a different liquid distributor outlet arrangement after the original design has been approved.

The modification may appear minor because only the outlet locations or diameters are changing.

However, the revised arrangement may affect liquid discharge behavior, distribution uniformity, outlet flow balance and operating turndown.

If manufacturing has already started, the modification may also require new materials, additional machining, revised welding details and repeat inspection.

The real risk is not the change itself. It is implementing the change without understanding its full engineering impact.

1. What Information Is Required for a Design Change Review?

Before evaluating a change request, the project team must establish the original design basis and the exact scope of the proposed modification.

Proposed Change Description

Clearly identify what is changing.

Possible changes include:

Equipment dimensions.

Distributor outlet diameter or spacing.

Nozzle location.

Support elevation.

Tray or packing arrangement.

Material specification.

Connection details.

Operating conditions.

Engineering documentation.

Avoid vague instructions such as "modify the distributor according to the latest requirement."

The requested modification should be defined through marked-up drawings, written specifications or clearly identified dimensional changes.

Current Approved Design

Identify the latest approved drawing revision and relevant calculation status.

The engineering team needs to know which design was originally approved and whether manufacturing drawings have already been released.

Affected Operating Cases

If process conditions have changed, provide the revised vapor and liquid rates, operating pressure, temperature and relevant physical properties.

Changes in process duty may require the hydraulic design basis to be reviewed.

Manufacturing Status

Confirm the actual production status:

Materials not yet purchased.

Materials already ordered.

Components cut but not assembled.

Welding completed.

Machining completed.

Inspection completed.

Equipment already packed or shipped.

The same design modification can have very different cost and schedule consequences depending on the manufacturing stage.

Required Approvals

Identify the parties responsible for technical, mechanical, quality and commercial approval.

A design change should not proceed solely because a revised drawing has been circulated by email.

2. Engineering Workflow: How to Evaluate a Design Change Request

A practical change management process should follow five steps.

Step 1: Register the Change Request

Assign a unique change reference.

Record the original drawing revision, proposed modification, reason for the request and date received.

This creates a traceable record for subsequent technical and commercial decisions.

Step 2: Perform an Engineering Impact Assessment

Identify all affected engineering disciplines.

Review whether the modification influences:

Process performance.

Hydraulic calculations.

Mechanical strength.

Equipment dimensions.

Vessel interfaces.

Installation requirements.

Material specifications.

Inspection procedures.

A change that appears mechanical may still affect hydraulic performance.

Likewise, a process change may require modifications to mechanical drawings and manufacturing details.

Step 3: Evaluate Procurement and Manufacturing Consequences

Review the materials and components already committed to production.

Determine whether existing parts can be reused, modified or must be replaced.

Assess additional machining, welding, assembly, inspection and documentation requirements.

Step 4: Revise Controlled Engineering Documents

Update all affected documents, which may include:

General arrangement drawings.

Fabrication drawings.

Hydraulic calculation reports.

Material specifications.

Bill of materials.

Inspection and test documentation.

Installation drawings.

Superseded revisions must be identified and withdrawn from active manufacturing use.

Step 5: Obtain Authorization Before Implementation

Confirm the required technical and commercial approvals.

The revised scope should clearly state the agreed modification, associated cost, schedule impact and updated deliverables.

Only the authorized revision should be released for the affected manufacturing activities.

3. Hydraulic Calculations: When Must They Be Repeated?

Not every drawing modification requires a complete hydraulic re-rating.

However, calculations must be reviewed when a change affects assumptions or equipment geometry that is important to the original hydraulic assessment.

Changes to Distributor Outlet Diameter

Changing an outlet diameter can affect the discharge behavior of individual outlets.

The resulting liquid flow distribution may differ from the original design.

A review should consider the available liquid head, outlet configuration, operating flow range and distribution requirements.

Checking the revised diameter alone is not sufficient.

Changes to Liquid Distributor Arrangement

Moving drip points, changing outlet spacing or modifying distributor sections may affect liquid distribution across the column cross-section.

The revised geometry should be assessed against the specified operating cases.

Changes to Tray Geometry

Changes to active area, downcomers, openings or other tray features may influence vapor-liquid contacting, capacity and pressure drop.

The affected hydraulic calculations should be reviewed.

Changes to Packing Support or Vapor Passage Area

Additional structural members or modifications to the support grid can restrict vapor flow.

The revised arrangement may affect pressure drop and available capacity.

Changes to Operating Conditions

An increase in vapor or liquid loading can change the original hydraulic design basis even when the equipment geometry remains unchanged.

The affected operating cases should be reassessed.

KEMVORI supports project-based hydraulic calculation and engineering review, including:

Vapor-liquid loading analysis.

Pressure-drop evaluation.

Flooding assessment.

Liquid distribution review.

Equipment sizing and arrangement assessment.

The required calculation scope depends on the nature of the change, available process information and agreed engineering responsibilities.

A hydraulic result should always identify its operating cases, equipment geometry, assumptions and calculation limitations.

4. Common Problems Caused by Poor Change Management

Manufacturing Continues with an Old Drawing

A customer sends a revised drawing by email, but the production team continues using the previously released version.

The result may be a fabricated component that no longer matches the approved design.

Prevention: Use controlled drawing revisions and formally notify manufacturing when an approved change affects production.

Hydraulic Calculations Are Not Updated

A distributor outlet arrangement is changed, but the original hydraulic calculation is still used to support the revised equipment.

The original calculation may no longer represent the actual geometry.

Prevention: Identify affected calculations during the change review and confirm whether re-evaluation is necessary.

Material Has Already Been Purchased

A material specification changes after procurement has started.

The original material may no longer meet the revised requirements.

Prevention: Review procurement status and material compatibility before approving the commercial change.

Mechanical Interfaces Are Overlooked

A support elevation or component dimension is modified without checking the associated vessel interface.

The revised equipment may create installation conflicts.

Prevention: Review all affected mechanical interfaces and installation clearances.

Cost and Schedule Impacts Are Not Communicated

Engineering approves a modification without assessing the additional manufacturing and inspection work.

The project then experiences unexpected cost and delivery delays.

Prevention: Provide a documented impact assessment before implementing the change.

5. Engineering Trade-Off: Early Changes vs. Late Changes

The timing of a design change can have a significant effect on project cost and schedule.

During preliminary engineering

Changes are generally easier to incorporate because detailed drawings and manufacturing activities may not have been finalized.

After drawing approval

Revisions may require additional engineering review, drawing updates and approval cycles.

After material procurement

Changes may result in material replacement, additional processing or unused inventory.

During fabrication

The project may require cutting, welding, machining, reassembly and repeat inspection.

After shipment or installation

Modifications can become substantially more complex because they may require transportation, site work, shutdown coordination or equipment replacement.

This is why design changes should be identified and resolved as early as reasonably possible.

However, rejecting a necessary technical modification simply to protect the schedule is not an acceptable engineering solution.

The correct decision should balance technical benefits, remaining project risk, actual manufacturing status and commercial consequences.

6. Illustrative Project Scenario

Consider a hypothetical packed-column retrofit project.

The EPC contractor initially approves the distributor arrangement based on the original process vapor flow rate.

After the distributor and packing sizing have been approved, the process team revises the design vapor flow.

The new operating condition may affect the original hydraulic rating.

Rather than simply updating the process datasheet, the engineering team registers a formal change request.

The review identifies:

The original and revised vapor loading.

The affected hydraulic calculation cases.

Whether the proposed equipment arrangement remains suitable.

Whether any distributor or support dimensions need revision.

The impact on fabrication and delivery.

The hydraulic assessment is repeated where required before revised installation drawings are issued.

This prevents the project from installing equipment based on an outdated process design basis.

7. Acceptance Checklist Before Implementing a Design Change

Before approving a tower internals modification, confirm the following:

The final change record should provide a traceable connection between the original design, approved modification, revised engineering documents and manufacturing release.

8. What Should EPC Buyers Include in a Design Change Request?

To help suppliers evaluate a modification efficiently, EPC buyers should provide:

Original approved drawings.

Marked-up drawings showing the proposed changes.

Original and revised process conditions.

Reason for the requested modification.

Relevant hydraulic calculation reports.

Vessel interface details.

Current manufacturing and procurement status.

Required implementation deadline.

Requested revised engineering deliverables.

The buyer should also clarify whether the supplier is expected to provide preliminary technical review, revised hydraulic calculations, detailed drawing updates, additional fabrication or repeat inspection.

This information helps establish the actual scope and avoids disagreements about responsibilities after the change has been implemented.

Frequently Asked Questions

Is a drawing mark-up enough to approve a tower internals design change?

Not when the modification affects hydraulic calculations, mechanical interfaces, material requirements or contractual scope. A controlled engineering review and formal technical disposition are required.

When must hydraulic calculations be repeated?

When the change affects the process assumptions, equipment geometry or operating conditions relevant to the original hydraulic rating.

Does every design change increase manufacturing cost?

No. Some changes can be incorporated before procurement or fabrication with limited additional work. The cost depends on the modification and actual production status.

Who pays for rework after a design change?

Commercial responsibility depends on the contract, the cause of the change and the agreed scope. The cost and responsibility should be documented before implementation.

Can manufacturing continue while a design change is under review?

Unaffected activities may continue when properly authorized. However, affected components should not proceed using an unapproved or superseded design.

Engineering Support from KEMVORI

KEMVORI Petrochemical Equipment Limited provides engineering review and customized tower internals solutions for distillation, absorption, refining, petrochemical and industrial gas-liquid separation applications.

Our engineering support can include:

Tower internals design review.

Engineering change impact assessment.

Hydraulic calculation support.

Vapor-liquid loading evaluation.

Pressure-drop assessment.

Flooding evaluation.

Liquid distribution review.

Mechanical interface coordination.

Revised equipment arrangement and fabrication support.

The exact engineering deliverables depend on project requirements, available process information and the agreed technical scope.

For an existing project design change, provide the original approved drawings, marked-up revisions, process conditions and current manufacturing status.

KEMVORI can review the available information and identify the affected engineering calculations, equipment interfaces and manufacturing requirements before the revised design is released.

Submit your engineering change request and drawings for technical review.

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