How can i give alphabets only validation in webform?
If you want to go for a module, Try the Webform Validation module

This module adds an extra tab to each webform node, allowing you to specify validation rules for your webform components. You can create one or more of the predefined validation rules, and select which webform component(s) should be validated against those. By using the hooks provided by this module, you can also define your own validation rules in your own modules.
The following validation rules are currently included:

Numeric values (optionally specify min and/or max value)
Minimum length
Maximum length
Minimum number of words
Maximum number of words
Equal values on multiple fields
Unique values on multiple fields
Specific value
Require at least one of two fields
Require at least one of several fields
Minimum number of selections required
Maximum number of selections allowed
Exact number of selections required
Plain text (disallow tags)
Regular expression
Must be empty (anti-spam: Hide with CSS)
Words blacklist
Must match a username