METHOD STATEMENT HVAC WORK

Contents:

1. Objective
2.  Scope
3. References
4. Material
5. Material Procurement, Handling and Storage
6. Duct Fabrication
7. Duct Installation
8. Flexible duct installation
9. Duct Insulation
10. QA/QC

1. Objective

This method statement provides information concerning the method of construction to ensure compliance with contract requirements, drawing, technical specification and quality control to provide a successful installation, testing, and commissioning of the Installation of Duct work.

• Scope

HAC shall supply and install in accordance with the specification of material and workmanship of HVAC duct system.

HAC shall complete install works, testing, witness testing at the site by the engineer, testing and commissioning of the HVAC duct system.

• References

• Contract Specifications.
• Material Submittal.
• Technical Literature for Manufacturer.
• Design Drawings.
• Shop Drawings.
• Latest edition of SMACNA Duct Construction Standards.
• SCG comments on HAC clarifications.

• Material

• G.I Duct for Supply, Return, Exhaust and Fresh Systems.
• Fittings shall be fabricated according to SMACNA.
• Joints shall be as per Pressure system and detail drawings.

• Material Procurement, Handling and Storage

• Upon approval of the material and shop drawing submittals, Contractor shall survey the QTY including fittings as per approved shop drawing and fabricate the material as per project schedule.
•   Upon delivery to site, all duct and accessories shall be stored indoor, in a clean, dry and ventilated place.

• Duct Fabrication

General:

• Ductwork sections shall be unloaded and stored in such a way to avoid damage and contamination.
• Duct shall be factory fabricated.
• Prequalification of duct factory shall be submitted and approved.
• Prepare approved shop drawing showing the size, fitting and transformation section of ducting.
• Determine quantity and correct gauges of G.I. duct before starting fabrication. The thickness of the G.I sheet shall be measured with the help of a Vernier caliper. 

Methodology:

• Before start duct fabrication, inspection shall for the galvanized coating shall be approved by an approved laboratory.
• Select the right thickness of material as per the constructional details.
• Removes coil set and cut the blanks of required length including the tolerance depending upon the size of the duct.
• The blanks are then passed through the grooving machine to strengthen the sheets.
• This is then passed through the lock former for preparation of joints.
• Bending machine is used to make `L’ or `U’ sections of the sheets. Square or rectangular sections are made in case of smaller ducts. The elbow via the 90 deg, 45deg and long radius tees are manually made based on the requirement as per drawing.

• Duct Installation

General:

• Ductwork routing shall avoid undue changes of direction, sets or bends.
• The internal joints shall be arranged such that raw edges do not oppose the airflow.
• Duct hangers are fitted at correct setting and location.
• Support spacing shall be relevant sections of SMACNA Duct Construction Standards, as per note on IFC details drawings. Ductwork support material shall be galvanized.
• Select the fabricated duct section with correct dimension and pairs.
• Where space conditions permit, full radius turns shall be used at offsets in low pressure ducts radius = duct width.
• Where space conditions do not permit a short radius, vaned elbow or square elbows shall be used. All square elbows shall be fitted with double thick turning vanes.
• All installation work shall be carried-out per approved drawings.

Methodology:

• The hanger rods and duct bracket angles are painted in three layers prior to installation.
• The fabricated sheets are boxed up to form ducts.
• Based on the size of the ducts, two/three sections of the ducts are joined (as indicated below) on the floor as per layout drawing.
• Two adjoining duct section are jointed together with slip joint, sealant is then applied over the corner of transverse joint.
• Prior installation, ducts are cleaned internally.
• The assembled duct pieces are lifted, seated on to the hangers and leveled, Seal application on seams and joints shall be continuous.
• The open ends of the ducts are closed with polythene sheets to avoid ingress of dust.
• The installed duct length is then offered for inspection. After the inspection clearance the duct length is pressure tested.
• All Joints shall be inspected and checked for maximum leakage allowance.

• Flexible duct installation

• Mark the opening for flexible duct collar connection on the surface of G.I. duct.
  • Cut the opening as required, install the flexible duct collar and then insert the flexible duct into the collar using adhesive.
  • Minimum length of flexible duct should be used. However, approved shop drawing to be referred to for the actual length based on site conditions. Duct should extend 100 mm beyond end of a sheet metal connection before bending and should not be compressed.
  • The saddles or hangers, which are to be used, must cover ½ the circumference of the outside diameter of the flexible duct.
  • Flexible duct supported at recommended intervals but at no greater distance than 3m as per 15321- Clause 2.5.1.
  • To prevent tearing of vapor barrier, do not support entire weight of flexible duct on any one hanger during installation.
  • Avoid contact of flexible duct with sharp edges of hanger material.
  • Damage to vapor barrier shall be repaired with tape.
  • Terminal devices connected by flexible duct shall be supported independently of the flexible duct.

• Duct Insulation

General:

• Insulating materials containing asbestos, PVC, polyurethane and polystyrene shall not be used.
• All materials shall be new and fully dried and must be maintained in this condition during the progress of the works.
• All insulating materials shall be proofed against rotting, mold, fungus growth and attack by vermin.
• All cements and adhesives shall be as recommended by the manufacturer of the insulation. Insulation, insulation jacket, canvas and adhesive shall be fire retardant with a flame spread not larger than 25 and a smoke developed rating not larger than 50 when tested in accordance with standard E 84-61 of the ASTM.

1. Methodology for Internal Duct Insulation/Lining (where applicable):

• Check that the external surface is cleaned and free from any dust … etc.
  • Measure and cut to shape the Duct Liner allowing a 5cm excess at downstream transverse joints.
  • Carefully remove insulation from 5cm excess leaving only the surface tissue for overlapping to next section.
  • Longitudinal joints should be close butted and sealed with a proprietary sealant, tape or metal angle.
  • Ensure that internal surface of duct is clean and free from dust and grease.
  • Affix mechanical fasteners in accordance with manufacturer’s instructions at not more than 45cm centers.
  • Fixings should start within 7.5 cm of joints corners, ends etc.
  • Apply suitable adhesive to duct surface before positioning Duct Liner over mechanical fastener pins.
  • Fit washers or caps to pins and snip off excess shank of pins.
  • Transverse joints should have 5cm tissue flap fixed to following section of liner with adhesive then overlay with metal strip fixed through insulation to dust sheeting by screws or rivets at 7.5 cm centers.
  • Start and finish ends of duct section to be treated as above with exposed edges of insulation coated with sealant or alternatively affix angle in place of metal strip with one leg masking edge or insulation.

• Methodology for External Duct Insulation:

• Ensure surface of duct is dry and free from dust or grease. Cut insulation to required length to fit around the duct making sure that edges are straight.
  • Apply insulation adhesive to duct surface in strips approx. 15 cm wide. Underside of duct should be completely coated with adhesive to prevent sagging.
  • Depending on size of duct fit 2 or more rows of insulation fixing pins to each face of duct at centers of 30 – 50cm. Pins can be self-adhesive type, bond-on type or stud welded. Carefully position insulation over the pins, pressing firmly so that pins penetrate through the insulation. Fit spring clip or lock washer over pins pushing them down until tight against insulation. Snip off any excess shank from pin. Seal all joints and openings with self-adhesive tape.
  • Wrap insulation, foil face up, to the prepared duct surface once adhesive has become tacky, or as instructed by manufacturer, making sure that all edge joints are closely butted.
  • Apply uniform hand pressure to outside surface of insulation to obtain good bond.
  • Tape all longitudinal and transverse joints with a good quality self-adhesive insulation tape. Seal any puncture points in insulation around hangers etc. with tape.
  • If lacing wire is used for additional security a light gauge galvanized angle should be fitted on each edge to prevent wire from cutting into insulation.

1. QA/QC
   1. Total air outlet volume for low pressure duct systems, measured by means of a velometer, shall be at least 95% of actual fan supply (measured by means of duct traverse taken with a pitot tube and water manometer.)

Total air outlet volume for medium pressure duct systems, measured by means of a velometer, shall be at least 98% of actual fan supply (measured by means of a duct traverse taken with a Pitot tube and water manometer) i.e., maximum 2% leakage.