CREATE FUNCTION getTotalWorkingSeconds(@machineList varchar(255))
RETURNS int
AS
BEGIN
  declare @res int
  Select @res =  Sum(DURATION) From PROCESSDATA where MACHINEID in (@machineList)
  return @res
END

SELECT dbo.getTotalWorkingSeconds('3,2')

result; Conversion failed when converting the varchar value '3,2' to data type int.

SELECT dbo.getTotalWorkingSeconds(''''+'3,2'+'''')

result; Conversion failed when converting the varchar value ''3,2'' to data type int.

CREATE FUNCTION Split
(
  @delimited nvarchar(max),
  @delimiter nvarchar(100)
) RETURNS @t TABLE
(
-- Id column can be commented out, not required for sql splitting string
  id int identity(1,1), -- I use this column for numbering splitted parts
  val nvarchar(max)
)
AS
BEGIN
  declare @xml xml
  set @xml = N'<root><r>' + replace(@delimited,@delimiter,'</r><r>') + '</r></root>'

  insert into @t(val)
  select
    r.value('.','varchar(max)') as item
  from @xml.nodes('//root/r') as records(r)

  RETURN
END
GO

CREATE FUNCTION getTotalWorkingSeconds(@machineList varchar(255))
RETURNS int
AS
BEGIN
  declare @t table (val nvarchar(100))
  insert into @t select * from dbo.split(@machineList,',')

  declare @res int
  Select @res =  Sum(DURATION) From PROCESSDATA where MACHINEID in (select val from @t)
  return @res
END