Table of Contents

  1. Purpose
  2. General
  3. Responsibilities
  4. Procedure
  5. References
  6. Attachments

1. Purpose

To provide the Process Design Engineer, or Process Department representative with a guideline for inspecting all trayed and packed towers.

2. General

2.1 A Construction Department representative should inspect all Company towers before and after the internals are installed, (to avoid discussions about causes of attained out of tolerances after installation of internals).

2.2 It is a standard Company requirement that in addition a Process Department representative should inspect such towers at the earliest opportunity. Possible non-conformance with the design requirements may thereby be identified, and remedial action taken as necessary.

Inspection by Construction (and Process) is necessary to ensure that the tower has been correctly erected. Additionally, inspection on site for 'non-conformance with design requirements', should be a formality if the tower was carefully inspected by Company Home Office representatives, prior to release from the manufacturer (see para 4.4).

Clients operations representative is invited to be present during this tray inspection.

2.3 The Process Manager may waive the requirement for a Process Depart­ment inspection by agreement with the Project Manager or Construction Manager.

3. Responsibilities

3.1 The Project Manager is responsible for advising the Process Supervisor when the Process Inspection is to be scheduled.

3.2 The Process Manager or Process Supervisor is responsible for assigning a Process Engineer or other Process Department representative to carry out the inspections.

3.3 The Process Engineer or Process Department representative is respon­sible for carrying out the inspections, and issuing the detailed inspection report. The inspection report should include a summary of remedial action to be taken and should be circulated with at least the following distribution:

  • Project Manager
  • Construction Manager
  • Manager of Start-up Operations
  • Process Manager/Supervisor
  • Client's Process Representative.

3.4 The Construction Manager is responsible for ensuring that all remedial actions requested are completed and inspected.

4. Procedure

4.1 After Construction and Process departments have completed their inspections, the column remains accessible for pre-commissioning and Client's operations group to carry out their checks:

Internal manways are left open and column manholes are closed with four bolts. Only temporary gaskets should be used until final closure.

When the trays have no internal manways, pre-commissioning engineer and the Client will check the trays during site installation.

Internal manways and column manholes to be closed as soon as a closing certificate is signed by all parties, (see attachment 4).

Demisters, if any to be installed during pre-commissioning. In certain instances, these will be installed after pickling, degreasing or flushing activities.

All packing to be installed during pre-commissioning.


Before entering any vessel, column, tank or confined space, ensure such entry is safe. When required, obtain an entry permit.

Inform the Site Safety Manager, or his delegate, about your entry. See also Company work procedures CO-SP-501 Exhibit I - Enclosed Space Entry Permit.

The following minimum requirements will be observed prior to entry:

  • Oxygen level measurement.
  • Position a person outside the column during the internal check.
  • Wear protective clothing and gloves.

4.2 Tray Inspection Criteria

Distillation and other tray type mass transfer devices suffer from deteriorating performance as the result of different liquid depth on the active sections.

Devices such as bubble cap trays will be less upset than vacuum tower sieve trays as the result of this effect due to greater liquid depth, but in both cases, exceeding the tolerances generally recognized as applicable to towers of various diameters, will provoke poor performance and jeopardize the company's guarantee position.

Tray inspection addresses two elements:

  • Mechanical integrity and robustness of tray installation. Check that bolting and sealing is tight and that the superficial condition of tray components is in good condition.
  • A measurement verification exercise to establish the degree of conformity of the installation.

The various contributors to tray installation faults are as follows:

  • Tray verticality, affecting;
  • tray support ring levelness.
  • Downcomer weir levelness as well as fabrication errors for these items, plus
  • buckling and out-of-flatness of the tray panel.

The tolerances in the table below are very similar to those used by tray fabricators so that disputes over measurements will be minimized.

Maximum out-of-flatness or out-of-levelness:

  • Towers upto 2000mm dia 3mm
  • Towers over 2000mm dia 6mm

This is an absolute levelness criterion as clarified below:

  • One criterion only is evaluated namely the absolute deviation of any point on a tray from a reference.
  • The method calls for measurement of the relative level of five points in each tray panel.
  • Every tray is measured using a single reference point for each pass side.
  • A set of calibrated measuring bottles (water level) with a suitably indented base connected by a flexible hose with a length not less than two thirds of the tower diameter is used to take readings which are subsequently analyzed in the inspection report.
  • The tray installation is classed as satisfactory if not more than 5% of all maximum deviation measurements for a single tower exceed the levelness tolerance by not more than 1 mm.

4.3 Preparation

In order to assist the process or pre-commissioning engineer in inspecting the trays, the checklist indicated in 4.4 will be used.

Beside the items mentioned on this list, the following specific checks will have to be carried out:

  • A leak test shall be performed on all total draw-off trays. With all drain holes plugged, the drop in level shall not exceed 5 cm in 30 mins. unless otherwise specified.
  • A spray pattern test shall be performed on high vacuum towers. This is to make sure that no spray nozzle is blocked and that the spray pattern is such that it covers the total diameter of the column.

Leak test procedure for draw-off trays

  • Block drain holes with putty.
  • Fill with water (check water quality - maximum chlorides).
  • Mark level and leave for 30 minutes; level shall not drop by more than 50 mm, unless otherwise specified.
  • Allow water to drain and check flow (slope of gutters/trough).
  • Remove all putty from drain holes.
  • Check cleanliness and arrange acceptance.

Spray pattern test procedure

A spray pattern test must be well coordinated. To perform this test the tower must be trayed, tray and column manways opened and the following made available:

  1. A circulation pump, preferably one of the process pumps.
  2. A copious water supply.
  3. Sufficient instrumentation to monitor the pump's performance, level and flow measurements:
  • A positive head must be established and the pump suction and discharge must be filled and vented.

If necessary, a pressurized hose should be connected to the suction to ensure a continuous supply of water.

  • A secure place above the distributor must be found to stand on and check that the spray covers the total surface.

The sequence of steps for spray testing are:

  • Remove all spray nozzles.
  • Provide all pump strainers and filters with start-up mesh (fine mesh) to ensure that no debris enters the system.
  • Flush the spray distribution and associated piping system with water.
  • Check the filters and re-install all spray nozzles.
  • Pump water through the spray nozzles at flow rates ranging between 50% and 150% of the design rate (unless otherwise specified).
  • Record the pressure at the inlet of the spray manifold for each flow rate.
  • Stop water flow. The observers enter the column and position themselves above the spray manifold.
  • The water flow is re-started to verify the performance of the spray nozzles.


  • Obtain an entry permit and results of the oxygen test.
  • Check for ventilation (water, mist and breathing).
  • Secure yourself with a safety belt.
  • Position a person outside the column (for radio contact).
  • Wear rain gear.
  • Carry a tool to give sinals against wall.

Start pump and stop as soon as result is noted. Client should be present in the column to confirm findings.

4.4 Process Check List for Columns/Vessels

The checks marked with (*) must be carried out at the manufacturers shop, if the tower is to be trayed prior to shipment. This does not preclude the necessity for site checks.

1. Check requisition/Vendor Drawings

  • Check for discrepancies on critical dimensions.

2. Check at Liquid Flow Path

  • Is liquid always present behind an inlet weir?
  • Are liquid cut outs positioned correctly?
  • Does liquid fall into correct tray position?

3. Check Liquid Feed Inlets

  • Are number and size of holes correct?
  • Is liquid distributed evenly over the tray?
  • Will there be any mal-distribution?
  • Check orientation of sparger and holes.
  • Check provisions for expansion differences.

4. Check Liquid Outlets

  • Check if vortex breakers are installed and do not obstruct flow.

5. Check Reboiler Returns

  • Does liquid enter correct compartments?
  • Does vapor distribute uniformly?
  • Does vapor pass through a curtain of liquid?

6. Check Thermocouple Positions

  • Are they located in 'dead' zones?
  • Do they obstruct liquid flow paths?
  • Check length of thermo couples.

7. Check Pressure Gauge Positions

  • Does liquid hit branch take off (particularly in vapor/liquid zones). If so it may need shielding.

8. Mechanical

  • Prior to start installation, a dimensional check shall be performed on all supports and tray rings. (*)

- Random checks for any malfunction of valves.

  • Random checks for any loose bolts, (check if tack welds are required). (*)
  • Check if gasket material is installed. (*)
  • Check number of trays/valves. (*)
  • Check drain holes in seal pans. (*)
  • Check position and dimensions of slotted holes. (*)
  • Check DC clearances per vendor drawings (should be same over the total length). (*)
  • Check weir heights per vendor drawing (should be same over the total length). (*)
  • Check provisions for expansion (beams, valve type inlet devices, spargers, distribution etc.). (*)
  • Check for levelness (important on vacuum towers with minimum tray liquid depth). (*)
  • The out of flatness and out of levelness of trays after installation should be within following tolerances: (*)

Be less than 3 mm for trays of up to 2000 mm wide and less than 6 mm for trays over 2000 mm wide, expressed as the difference between highest and lowest points on the tray.

Note: For partial draw-off trays with a small liquid transfer to the lower trays, also a leak test may be considered.

9. Attachments

  • All internal attachments, e.g. bolts, nuts, clamps, blanking strips, support brackets, etc., should be securely fastened. (*)
  • All internal nozzles should be tightly flanged, if applicable. (Note weep and vent holes if any. On shutdown all liquid should be able to drain freely).(*)

10. Stiffness

  • Check the stiffness of weirs, down-comers and other unsupported attachments to ensure deflection/vibration will not occur. (*)

11. Free Movement

  • Where adjustable weirs have been specified check that such weirs are secured at the correct height and that future adjustment is possible. (*)

12. Materials

  • Check that all materials are in accordance with specification. It is often useful to carry a magnet during inspection to check for rogue carbon steel bolts, clamps, brackets etc., in stainless steel (austenitic) service. (*)

13. Leakage

Particular attention should be paid to sealed areas for possible leakage:

  • All weirs should have a good all-round seal.
  • The tray itself should be checked for a good seal between the different sections and a proper gasket between the tray and the support ring.
  • Seal pans should be inspected for a good seal.

14. Miscellaneous Internals

  • Special internal items should be identified and checked out, e.g. Vortex breakers, anti-swirl baffles, demister pads, etc. (*)
  • Where sieve trays have punched holes, check if the direction of punching is in accordance with specification, (usually down). (*)
  • Where holes have been burned into a tray, for example with valve or bubble cap trays, all burrs and slag should have been com­pletely removed. (*)
  • Check that the support ring, or other internals, do not blank sieve holes, valves etc. (*)
  • Check that the trays are clean. No bolts, nuts or debris should be left on trays.

15. Packed Columns

The packing should be checked prior to loading to ensure the correct type of packing has been received. Check:

  • Type (Pall, Intalox, etc.).
  • Size.
  • Material.
  • Try to assess damage, if any, (e.g. ceramic packing).

It is useful to keep a sample of packing for future reference.

In case of structured packing, check if sealing against wall, gap between packing elements, wall scrapers, number of layers, type etc., is as per specification

  • Check net free area of packing supports and hold-down grids. (*)
  • Check if the ancillary internals (support tray, distributors, re-distributors, etc.) have been correctly installed. (*)
  • Ensure that correct method of loading is used - wet, stacked, random etc.
  • Check correct height of packing and where specified the hold down grids.

16. Distributors

  • Distributors should be thoroughly checked for levelness with a graduated water level in a similar manner to the tray level check. (*)
  • Check distributor piping for weep and vent holes where applicable. (*)

17. General Appearance

The general appearance and cleanliness should be checked and reported. Note that all debris and as much loose rust, slag, etc. as possible should be removed prior to start-up to avert possible problems.

18. Report

The Process department representative should make sure that all details of such tower inspections are fully reported since this inspection is usually the last opportunity to collect this data in a complete manner. In case of performance problems in a tower reference will be made to this original pre-startup inspection report. details should therefore be as complete as possible.

19. Punch List

When discrepancies are discovered, they will be recorded on a punch list.

This punch list will be cleared by the Construction group.

5. References

5.1 Job Specification for Pressure Vessels.

5.2 Job Specification for Fractionating Trays and Components.

6. Attachments

  1. Construction Tolerances for Pressure Vessels
  2. Process Check Inspection Sheet
  3. Process Inspection Certificate
  4. Closing Certificate


1. Construction Tolerances for Pressure Vessels

(not available on Internet).


2. Process Check Inspection Sheet

Tower No. Name :  
No. of Trays : Type :  
1.0 Tray No. Type :
2.0 Number of Holes, Valves, Bubble Caps, Chimneys :
2.1 For Sieve Trays :
Diameter of Chimney in mm :
Hole pitch in mm :
Distance Chimney Top - Cap in mm :

Calming Zones :

At inlet weir : Length of calming zone in mm :
At outlet weir: Length of calming zone in mm :

Inlet Weir :

Length in mm :
Level Deviation in mm :
Height of inlet weir in mm :


Outlet Weir :

Length in mm :
Level Deviation in mm :
Height of outlet weir in mm :

6.0 Downcomer Clearance in mm :
7.0 Levelness/Flatness of Tray :
(Deviation of level in mm) :
Along Flow Path :
Perpendicular to Flow Path :



Nozzles :

Inlet Nozzle :
Height of bottom of nozzle above tray in mm:
For Sparger :
Diameter of Hole in mm :
Number of Holes :
Pitch in mm :

Outlet Nozzle :
Height of bottom of nozzle above tray or gutter,
etc. in mm :

9.0 Seal Pan :
Weir height in mm :
Downcomer clearance in mm :

Remarks : e.g.
Have drain holes been provided in seal pans?
Have vent holes been provided in spargers?
Do trays require cleaning?

Inspection Carried Out by:


3. Process Inspection Certificate


4. Closing Certificate