my method

        string url = "http://www.pratikev.com/fractalv33/pratikEv/pages/viewProduct.jsp?pInstanceId=3138821";

            var hlWeb = new HtmlWeb
            {
                AutoDetectEncoding = false,
                OverrideEncoding = Encoding.GetEncoding("iso-8859-9"),
            };
            HtmlDocument myDoc = hlWeb.Load(url);

Your method

        HtmlDocument doc = new HtmlDocument();
        using (var response = (WebRequest.Create(url).GetResponse()))
        {
            doc.LoadHtml(new StreamReader(response.GetResponseStream()).ReadToEnd());
        }


At my method your xpath returns null