Keystone Filter a division of Met-Pro Corporation is headquartered in Hatfield, Pennsylvania, which is approximately 30 miles northwest of the city of Philadelphia. Keystone Filter manufactures a wide range of air and liq......Keystone Filter a division of Met-Pro Corporation is headquartered in Hatfield, Pennsylvania, which is approximately 30 miles northwest of the city of Philadelphia. Keystone Filter manufactures a wide range of air and liquid filtration products for industrial, commercial and residential markets. These filter products include industrial liquid filters and home drinking water filters, filter cartridges and housings. Keystone Filter is also one of the premier custom pleaters and custom filter cartridge manufacturers in the United States