Process engineers are not only responsible for the most economical design and operation, they are also responsible for the safe design and operation of equipment. This chapter deals with the fundamentals of process safety.

Threshold Limit Values(TLV)

The threshold limit values are the highest concentration of a toxicant in air that can be tolerated for a given length of time without any adverse effect. There are five types of TLVs.

1. TLV-TWA

   The time weighted average of a toxicant for a normal eight-hour work day or 40-hour work week to which a worker may be repeatedly exposed, day after day, without adverse effect.

2. TLV-STEL

   The short term exposure limit is the maximum concentration to which a person may be exposed continuously up to 15 minutes without adverse effect.

3. TLV-C

   The ceiling limit is the concentration which should not be exceeded any time.

4. PEL

   The permissible exposure limit is the maximum concentration exposure allowed by OSHA 29CFR 1910.1000

5. IDLH

   The concentration of the toxicant that is Immediately Dangerous to Life or Health is the maximum concentration from which one could escape within 30 minutes without escape-impairing symptoms or any irreversible health effects.

TLVs are expressed either in parts per million by volume(ppm) or milligram per cubic meter(mg/m ³). The relation between ppm by volume and mg/m ³ is as follows.

Where:

P = pressure in atm., T = temperature in K, M= molecular mass.

Calculation of Time Weighted Average for Different Exposures to Same Toxicant

If a worker is exposed...