There are two ways:
1) [preferable] - in html form combo box - set the value of option tag as the AlphaNumber from TableA. So, when ever form submits - it will send the value of selected option from combo box - so, it will pass the AlphaNumber instead of displayed text. This way, your stored procedure will remain as it is.
2) Change the SP as:
ALTER Procedure [dbo].[spLocal_InsertValidationData]
@TagName Varchar(50),
@ValidTime datetime,
@Value real,
@Reason1 varchar(50),
@Reason2 varchar(50),
@Reason3 varchar(50),
@Reason4 varchar(50),
@Comments varchar(50),
@Name varchar(50),
@ValidAPMId numeric,
@ValidPMId numeric,
@InterfaceFlag bit,
@OOSpecFlag bit
AS
Declare @ValidOPId int
Select @ValidOPId = AlphaNumber From [TableA] Where [Name] = @Name
INSERT INTO ValidationDetail
VALUES
(@TagName,@ValidTime,@Value,@Reason1,@Reason2,@Reason3,@Reason4,@Comments,@ValidOPId,@ValidAPMId,@ValidPMId,@InterfaceFlag,@OOSpecFlag)