AP Petty cash type invoices needs to be approved by another user

We have defined the AP approval setup based on the Invoice amount limits as follows.

0>30 000 Asst Accountant

30000 >50000 Accountant

<50000 Finance Manager

But only Petty cash invoices needs to be approved by another approver and not based on the above limits.We have defined a DFF in invoice header to capture the additional details in petty cash invoices.

Please let us know a method to satisfy above requirement.\

Thanks in advance.