The Sounders grabbed the lead in just the 10th minute of play when Eddie Johnson nodded home a corner kick from Mauro Rosales.
The Union came close to equalizing on two occasions in the first half when Jack McInerney's header came back off the crossbar and Amobi Okugo nodded a shot off the post.
They struck the woodwork again in the 50th minute when Conor Casey's header grazed the post and came back into play, but Danny Cruz capitalized on the fortuitous bounce by roofing a shot into the upper corner from close range.
Cruz completed his brace just a minute later by cushioning an aerial ball with his chest and blasting a half-volley into the bottom corner of the net.
Philadelphia held the lead until the 64th minute when a cross from Leonardo Gonzalez was guided into the far side of the net by Rosales from the center of the penalty area.
The match turned gritty in the final 10 minutes as both sides battled for a winning strike. Lamar Neagle and Sheanon Williams were issued red cards for their involvement in a small fracas while DeAndre Yedlin was sent off in stoppage time for a rash challenge on Michael Farfan.
The Union had their appeals for a late penalty fall on deaf ears after Casey's header appeared to be denied by a Sounders handball, leaving the two sides to settle for a point.
05/04 18:31:09 ET