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SEMI S2-0703a E © SEMI 1991, 2004 40 APPENDIX 3 DESIGN GUIDELINES FOR EQUIPM ENT USING LIQUID CHEMICALS — Design and Test Method Supple ment Intended for Internal and Third Party Evaluation Use NOTICE : The material in t…

SEMI S2-0703a

method is to make two traverses across the diameter of

the duct at right angles to each other. Reading are taken

at the center of annular rings of equal area. Whenever

possible, the traverse should be made 7.5 duct

diameters downstream and 3 diameters upstream from

obstructions or directional changes such as an elbow,

hood, branch entry, etc. Where measurements are made

closer to disturbances, the results should be considered

subject to some doubt and checked against a second

location. If agreement within 10% of the two traverses

is obtained, reasonable accuracy can be assumed and

the average of the two readings used. Where the

variation exceeds 10%, a third location should be

selected and the two air flows in the best agreement

averaged and used. The use of a single centerline

reading for obtaining average velocity is a very coarse

approximation and is not recommended. If a traverse

cannot be done, then the centerline duct velocity should

be multiplied by 0.9 for a coarse estimate of actual

average duct velocity. Center line duct velocity should

not be used less than 5 duct diameters from an elbow,

junction, hood opening, or other source of turbulence.

A2-8.2 For ducts 150 mm (6 in.) and smaller, at least 6

traverse points should be used. For round ducts larger

than 150 mm (6 in.) diameter, at least 10 traverse points

should be employed. For very large ducts with wide

variation in velocity, 20 traverse points will increase the

precision of the air flow measurement.

A2-8.3 For square or rectangular ducts, the procedure

is to divide the cross-section into a number of equal

rectangular areas and measure the velocity pressure at

the center of each. The number of readings should not

be less than 16. Enough readings should be made so the

greatest distance between centers is less than 150 mm

(6 in.).

A2-8.4 The following data are required:

A2-8.4.1 The area of the duct at the traverse location.

A2-8.4.2 Velocity pressure at each point in the traverse

and/or average velocity and number of points measured.

A2-8.4.3 Temperature of the air stream at the time and

location of the traverse.

A2-8.4.4 The velocity pressure readings obtained are

converted to velocities, and the velocities (not the

velocity pressures) are averaged. Useful equation: V =

4.043 (VP/d)

0.5

, where V = velocity in m/s, VP =

velocity pressure in mm H

O, and d = density

correction factor (unitless). Some monitoring

instruments conduct this averaging internal to the

instrument.

A2-8.5 Flow measurement taken at other than standard

air temperatures should be corrected to standard

conditions (i.e., 21C [70F], 760 mm [29.92 in.] Hg).

SEMI S2-0703a

APPENDIX 3

DESIGN GUIDELINES FOR EQUIPMENT USING LIQUID CHEMICALS

— Design and Test Method Supplement Intended for Internal and

Third Party Evaluation Use

NOTICE: The material in this appendix is an official part of SEMI S2 and was approved by full letter ballot

procedures on December 15, 1999 by the North American Regional Standards Committee.

A3-1 Introduction

A3-1.1 This appendix provides specific technical

information relating to Section 23. In general, it

provides information on potential hazards,

recommended control methods, and design

considerations.

A3-1.2 This appendix is not intended to limit hazard

evaluation methods or control strategies (e.g., design

principles) employed by manufacturers. Alternative

methods are acceptable if they provide an equivalent

level of hazard control.

A3-1.3 This appendix is intended to be used as a

starting point for reference during equipment design.

An example would be during a formal hazard analysis

in a brainstorming session.

Table A3-1 Liquid Chemicals

Potential Hazard Recommended Control Method Design Considerations

Containment, control, and alarm

notification for spills, leaks or vapors.

Appropriately sized secondary containment (minimum 110%

volume of entire contents)

Equipment exhaust

Leak sensors to initiate auto shutdown.

Controlled access to chemical

containment areas.

Door/access cover interlocks that automatically depressurize the

area of the system being accessed.

Exposure to

operators

Control of access to point-of-

operation hazards.

Physical guarding/presence-sensing devices

Control of chemical delivery

pressure; control of residual

chemicals.

Depressurization upon system failure, interlock activation, or

normal shutdown

Transparent doors/covers allow visual inspection.

Exposure to

maintenance

personnel

Serviceability Built-in system purge and flush capabilities

System components accessible and easy to service.

Chemical resistance/compatibility

Appropriate materials used for equipment construction and

components.

General

equipment and

component

failure

Pressure rating

Pressurized systems designed to withstand 150% of maximum

foreseeable pressure, or provide a suitable relief valve.

Durable bulk chemical containers

Use of approved (e.g., DOT, UN Dangerous Goods) containers

in bulk distribution systems.

Control of pressurized vessels and

piping.

Provide visual pressure indicators with or without alarms.

Pressurized vessels and piping are designed and built to

recognized standards.

Spill control Automatic system pressure check prior to allowing dispense.

Use of normally closed valves on distribution lines.

Chemical

delivery system

leak

Drum change-out controls Over-fill sensors on chemical baths

Monitoring for excess flow.

Keyed and color-coded quick-connects

SEMI S2-0703a

Potential Hazard Recommended Control Method Design Considerations

Control of ignition sources.

NFPA 70 (NEC) Class I, Div. 2 wiring methods, intrinsically

safe components, or nitrogen-purged enclosures

Physical separation of ignition sources and/or potentially

flammable atmospheres.

Use of low voltage to reduce the risk for ignition.

Control of static electricity (i.e., one

type of ignition source).

Maintain ground continuity

Fire

Heat/fire/chemical detection

Limiting concentrations of fuels and

oxidizers.

(No consensus for a specific recommendation at the time of

publication of this guideline.)