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                                                                   EXHIBIT 10.21



                         AMENDED SINGLE INSTALLMENT NOTE
 HOUSTON, TEXAS  (EXERCISE OF OPTIONS FOR 565,741 SHARES OF NTG)  JULY 1, 2002

This Amendment pertains to that certain Single Installment Note entered into on
or about October 28, 2000 by and between Nathaniel A. Gregory and NATCO Group
Inc. ("NGI") for the funds necessary to facilitate the exercise of certain stock
options. The original note was in the principal amount of $1,222,381.73 with
interest accruing at an annual rate of 7.8% with payment of the principal
balance and all accrued interest due on or before July 31, 2003.

On July 1, 2002, NGI and Gregory agreed to amend the original note and Gregory
agrees, as follows:

         In consideration of value acknowledged and received, Gregory agrees to
         pay NGI the principal amount of $1,381,987.94 (i.e., $1,222,381.73 in
         initial principal plus $159,606.21 of accrued interest) on which
         interest shall accrue at an annual rate based on ninety-day LIBOR plus
         300 basis points adjusted on the first day of each calendar quarter.
         All payments of principal and interest on this Amended Note shall be
         paid in the legal currency of the United States; and Gregory waives
         presentment for payment, protest, and notice of protest and nonpayment
         of this Amended Note.

         Payment of the principal balance and all accrued interest shall be due
         July 31, 2004. However, Gregory reserves the right to repay this
         Amended Note, in whole or in part, before July 31, 2004 with no
         prepayment penalty.

         Failure to make payment when due shall be considered default. Should
         default exist for more than a grace period of sixty days, Gregory
         agrees to pay NGI all reasonable costs and expenses incurred during
         collection proceedings including, but not limited to, attorney's fees.

         No renewal or extension of this Amended Note, delay in enforcing any
         right of NGI under this Amended Note, or assignment by NGI of this
         Amended Note shall affect the liability or the obligations of Gregory.
         All rights of NGI under this Amended Note are cumulative and may be
         exercised concurrently or consecutively at NGI's option.

         If any one or more of the provisions of this Amended Note are
         determined to be unenforceable, in whole or in part, for any reason,
         the remaining provisions shall remain operative.

         Gregory acknowledges and agrees that this Amended Note is with rights
         of full recourse and that he is responsible for repayment of the
         principal and all accrued interest on or before the maturity date.

         THIS AMENDED NOTE SHALL BE CONSTRUED, GOVERNED, AND INTERPRETED IN
         ACCORDANCE WITH THE APPLICABLE LAWS OF THE STATE OF TEXAS.


                                             /s/ NATHANIEL A. GREGORY
                                            -----------------------------------
                                            Nathaniel A. Gregory

State of Texas   )
County of Harris )

         SWORN TO AND SUBSCRIBED TO before me on this 30th day of July 2002, by
Nathaniel A. Gregory, a person known to me.


                                          /s/ PATRICIA G. SMITH
                                         --------------------------------------
                                         Notary Public for the State of Texas


         (S E A L)
                    PATRICIA G. SMITH
         [SEAL]   MY COMMISSION EXPIRES
                    AUGUST 14, 2002